Chills

That is also the problem.  What percentage of the 4% of the market want Affinity or any other program.  
Even for developers, a lot of   professional tools that are available on Windows and mac are not on Linux.
For many of the reasons discussed here.
If 10% of the Windows users want Affinity, that is 9% of the market.
If 10% of Linux users want Affinity, that is 0.35% of the market.
The difference is the cost of developing for Windows is far lower per seat than developing for Linux.
It is also a far smoother, and predictable path. A LOT less commercial risk.
 

The problem is that on the desktop, Linux is NOT a major environment. It has less than 4% of the market.

Having worked in critical systems' software for about 35 years, I can tell you Linux has major, serious, problems. 
Firstly, it is a religion.  Its followers evangelize and make exudes for problems  The Faithful accept these excuses, and tend to double down, but in the cold light of day they don't stand up. Just because it is mostly POSIX compliant does not make it a UNIX.   There are POSIX systems that are SIL 3 rated and Do178 DAL A rated. Linux is usually not permitted for any 61508 or Do178 system.

Second, it is very easily intentionally corrupted (and often is).  Apart from accidental problems, hackers, up to state level actors, have intentionally put malware and other problems into Linux. Yes it could be done with other OS but it is far more difficult and would requite the knowledge of the companies involved. In the case of Apple and MS, only the US government could do this.  With Linux, almost anyone can do it. (see the Thompson UNIX back door hack for a method) Before COVID there were found to be multiple ghost contributors to the Linux Kernel. No one knew who they were, and some of their patches did affect kernel security. I believe three were thought to be state actors.
Thirdly, much of the code is appallingly bad. (I have seen good static analysers run over the code)  As Linus Torvald has said, testing is not good and people are adding "cool" things rather than writing solid code. Patches go in without proper regression testing, and neither do they tend to test anything bar the patch.
Next, Almost every Linux is different to the next one. There is no standard distribution.  Most Linux web servers are based on a commercially maintained distribution from a major commercial Linux distribution. The Linux I run here is a version that has been customized, tested and maintained by a commercial company.  It is not let back out into the wild. 

Linux has a very long way to go before it is as robust and safe as other systems.

Other than the POSIX interface Linux and UNIX are completely different. Hence the debate between Tanenbaum and Torvolds.  As you know Tanenbaum did Minix.  The debate is fully public.  The fundamental differences are why Linux isn't used in critical systems. Linux mostly isn't  POSIX compliment. it also doesn't work like UNIX. Macs are based on BSD via NextStep. The problem is most Linux are different to each other and constantly changing.

Most of my 40 years in SW has been below the OS API   I normally work with RTOS, often bare metal.  Though I used Solaris as a development host to target various other things. (PPC, 68K etc)  and have run many OS on the desktop, CP/M,  OS9, UNIX(various), LynxOS, Dos (various) Windows, OSX, and several Linux.
Serif as major developers will have the inside track at both Microsoft and Apple on where the OS is going and the Beta's long before the public see them. Usually a year or more in advance. They will also have a hotline to the dev team and support.
With Linux there are 100s of teams developing lots of different Linux to no particular plan. (many come and go, there are over 1000 obsolete Linux distributions) Serif and MS/Apple will sign mutual NDA's  Where do you do to sign an NDA for "Linux"?  I don't mean the kernel but the distribution? Where do you get the guaranteed  plan for Linux for the next couple of years?  No one develops SW like this for an unknown moving target
This is why BMD for Resolve went with ONE Linux distribution (right down to a version number)  from a major Linux distributor who was a legal entity they could sign legal agreements with. If you used *any* other Linux BMD said you were on your own.

So you are asking Serif to put in a LOT of work for a version of Affinity on Linux for a very small numbers of users.  The number of Linux users is  apparently around 4% of the market of Linux, MS, Mac users.  Most of them won't want Affinity. Serif won't get their money back.   They are in business not religion.
 
 

I said there were over a thousand Linux distributions.    I know a dozen that are not listed on the Linux family charts.   I also recall seeing a list 500 obsolete and unsupported Linux distributions.  
When Linux first started, every man and his dog was doing Linux distributions.  Some for things like the 8** series PPC parts. Though these were somewhat re-engineered.
Resolve had its own Linux (that only worked with its own hardware)   The problem depending how you build it with which versions of which libraries and which apps and drivers, often no two are the same.  

I did have the inside track on one company that puts its app onto a Linux host.  They also have hardware to debug a target running Linux. They automatically add 10% to the list price for anyone targeting Linux if they are a new customer. Simply to cover the additional support they will have to give. 
It simply isn't worth Serif supporting Linux, the market is too small and too fragmented.
 

I am now all in with creative cloud.  I will check back on occasion to see what is going on but for now the Affinity apps are gone from my system.
Had there been a reasonable LR alternative I would have stayed but that is not going to happen...

The difference is Adobe has a very good RAW converter. Affinity doesn't (yet)
Then again for a catalogue program what you really need a good database system. (Unfortunately, SQLite isn't particularly good.)
However, in principle you are correct. Take modules from A Photo add a database and a UI....
With the interface to A-Publisher, it will be a winner for people like me who do magazines and books.
Where you have the "Stock" interface to Pexels and Pixbay just add C-Cat. A winner.
 

Although I'm not a programmer, I briefly worked in the I.T sector in the early 2000's (Tech support for Symantec as well as web design in the early days of Wordpress).
Adobe acquired what is now Lightroom many moons ago and rebranded it (I use to use it's original iteration back in the day).  From what I can tell, Lightroom uses base core functionality of Photoshop.  Affinity already has AP developed and it would seem to me at least that integrating certain core features from AP2 and combining an SQL Lite engine for the cataloging aspect would seem fairly straight forward (But I could be completely wrong).  Again, I am no programmer but given all that I've seen in this field as a photographer for over 35 years now it seems that a committed group of coders could make a LR/C1 type app a viable reality. All it takes is enough financial resource and maybe that's the roadblock for Serif at this time. 

I create one master page for the basic stuff like footers and headers and then create other master pages with the layout stuff like text frames. Then I apply the basic one to the layout ones.
You might want to consider using custom fields (aka user variables). You could create one for your volume and issue numbers, insert those into the header or footer, and then just change the custom field for each issue.
Cheers

I have just read the links above you supplied.  I can't see any commercial SW company distributing using flatpak. 
It raises a LOT of security issues and requires a lot of work to maintain. The security issues are not one a SW company would want to take responsibility for.
In addition, I can't see Flatpak covering all Linux distributions. In which case, you are better off selecting a mainstream Linux and only supporting that.
However, for a fraction of the 4% of the desktop market, it is not worth it.

As you can see in many walks of life, a small, but vocal, group of evangelists tend to drown out the 90% majority who are quietly getting on with it.
It is the same with Linux.  A small number making a lot of noise.  

How about master pages based on other master pages? Does this "I have multiple (…) page numbers set up" mean they were created on / copied to several master pages?  Unless you want changing styles or positions across the document just two would be sufficient.

Blindingly obvious only after I tell you about it, so not obvious at all.
Use one (or more) of these Fields, put the field in the appropriate Master Page(s).

There is also a Running Header Feature which may work as well but it is more complicated to achieve the same result.

Recently (November 2023) in another place, a video editor was giving his reasons to come off Adobe Sw.  Various replacements were suggested, including Affinity, for the usual apps and reasons. (also Resolve for Video Editing)
The ONLY thing people can't find a good replacement for is Lightroom. 
Lightroom is a photo-management tool.   A catalogue system.   If Serif do a photo-catalouge system that works with Photo it would be a winner.  Many , like me are still on Lightroom 6.14 which was the last stand alone perpetual licence before CC. Certainly, all the world's photo-journalists love Lightroom.
 

No thanks.

No.  UNIX was originally an operating system released by AT&T's Bell Labs.  Various forks of the UNIX code became the various UNIX platforms (Solaris, AIX, HP-UX, SCO, etc.) which continue on long past the death of the original UNIX (a few versions of which you can download for free now and run on PDP-11 emulators if interested).
In order to promote portability of applications among these and other systems the Portable Operating System Interface standard (POSIX) was developed, which various operating systems (including UNIX, Linux and even Windows at one time) have offered compatibility with, either as their core interface for their own native applications or as an alternative API to allow "portable" code to run on an otherwise proprietary system.
UNIX later became a "standard" that companies could be certified to use as a label for their platforms, with several of the traditional UNIX vendors paying the fees and meeting the requirements for certification (as macOS currently does).  These standards, like POSIX, relate to the programming interfaces and the command line environment and largely ignore any graphical desktop interface which may or may not be sitting on top.
Tanenbaum developed a microkernel operating system called MINIX which is largely designed for POSIX compatibility but which uses a microkernel architecture, which he has argued in defense of at various times.  I tend to agree that microkernels have major benefits over the more traditional monolithic kernels that most operating system platforms continue to use, but the same could be said in the other direction as well, with monolithic kernels having a different set of advantages.  Several of the benefits of a microkernel make this architecture superior as a teaching tool (when studying the source code) and MINIX was designed for exactly that: to be something that students could study and learn from.
Linux was a personal project Torvalds started as an experiment / learning opportunity of his own, but he opted to develop it as a monolithic kernel rather than a microkernel, which Tanenbaum (who Torvalds had been a student of) evidently took exception to and started those "debates" in an apparent attempt to steer his student back to what he saw as a preferable design (and was probably right).
Linux, like MINIX, was never based on UNIX source code, but follows many of the design principles and has a high degree of POSIX compatibility, in spite of having a very different underlying architecture from that of MINIX.
 
Note that the whole microkernel vs. monolithic kernel debate is largely tangental to the UNIX vs. Linux vs. macOS vs. whatever debate - it has nothing to do with whether or not something is "UNIX" or "Linux" or for that matter implements some version of the POSIX standards.

I am not sure whether that is true any more.
I see in the Resolve Forums many moving from Adobe.
I think it is true that the hobby, semi pro and some pro's are moving to the "cheap" options. However, many professionals (never mind the rest) simply don't want the Adobe Subscription model. 
 

Right! That's why Illustrator has been such a dismal failure for Adobe. 

Yes, you can install as many as you want. The links are available from this handy FAQ:
 

This is exactly what I recommended. Duplicate the en_GB (UK) dictionary files as the base language and rename to en_ME (or whatever you want as long as it's not another real locale). en_ME will appear as English Montenegro but you'll know it's English Medieval. Then with that language selected, when you add misspelled words to the user dictionary, they will be added to en_ME and not to your normal en_GB (UK) user dictionary.
This is because Publisher for Windows does not include en_CA, it includes only en_US (USA) and en_GB (UK). If you were to install the en_CA dictionary you wouldn't see a warning. And yes, Publisher can have dozens of languages installed an in use simultaneously.
That's exactly what the Character panel's Language > Spelling option does, it specifies the spelling language for the selected text. (There's a separate language option for hyphenation.)
I've explained this in detail in my free PDF manual for Publisher which you can download from this forum using the link in my signature.

Updated for Publisher 2.5 (May 2024)
Publisher Manual May 23 2024.pdf
This free in-depth manual is filled with steps, tips, and recommendations for:
Documents, Pages, Master Pages, Sections, and Baseline Grid Character and Paragraph Formatting Text Styles (paragraph and character) Text Frames, Text Flow, and Stories Images and Picture Frames Books and Chapters Cross-References, Table of Contents, and Index Notes - including Footnotes, Sidenotes, and Endnotes Fields - including Page Numbering, Running Headers, and Custom Variables Anchors and Hyperlinks Printing and Exporting Settings New for this edition: Getting Started, Variable Fonts, and more This manual does not cover illustration and photo editing.
Q&A
Is this really free? Yes, it's copyrighted only to prevent others from publishing it as their own Is there a longer manual I can purchase? No Is there a version for Designer or Photo? No, but many of the features are the same so it will still be relevant Is there an iPad version? No, the steps and screenshots are for the macOS and Windows versions Why does it use a print layout if it's a PDF? It's a demonstration of how to use the features it describes to create a print book You might also like:
Special Characters Quick Reference Chart: https://forum.affinity.serif.com/index.php?/topic/176386-special-characters-in-affinity-quick-reference-chart/ Terminology Translation Dictionary for Dutch, Polish, Ukrainian, Swedish, Norwegian, Danish, and Finnish: https://forum.affinity.serif.com/index.php?/topic/209953-terminology-translation-dictionary/ Previous 2.4 manual for those who still want it:
Publisher Manual Feb 26 2024.pdf
 

In Serif's original roadmap for the Affinity suite a Lightroom alternative was listed as something they wanted to do as the fourth program of the suite. That roadmap also mentions both Publisher and the IPad apps which at the time didn't yet exist. You can see this in the forward to the Affinity Designer workbook. 
My theory is that when developing Publisher and the IPad apps that both Affinity Designer and Affinity Photo took real hits to their development of new features. It's likely Serif felt that those programs had lost a lot of momentum that they originally had as they just weren't being developed fast enough for its user's needs and so they decided to refocus on their main three products for the time being until they are more feature complete. 
I have no doubt a lightroom alternative is coming eventually. But it does seem to have been delayed for the moment. 

It's not possible now. 
However, if it helps, you can Ignore a spelling rather than Learning it. The Ignore list is specific to a document, rather than being saved in a dictionary fine.

Lightroom does fit my needs, and I did pay for it.   I use Lightroom 6.41 the last Standalone perpetual licence version. I started with Lightroom V1   Adobe did say that Lightroom would continue like that rather than be CC and on subscription.   However, they changed their minds and stopped any support for the standalone version just before Google changed the API for Maps. So the whole Maps section of Lightroom V6 (or previous)  no longer works.   Perhaps I am just being cynical.  
I have asked Adobe, multiple times to fix the Maps in the last standalone version.  I, and I suspect most other users, would pay for a one off upgrade to V6.2 with the maps fixed.  Otherwise, I am looking for a non-Adobe alterative.
I pay for good tools.  I paid for Lightroom, I paid for Affinity  (both V1 and V2 universal licences), I paid for the Studio version of Resolve. I paid for DXO (and updates)   What I won't do is pay a subscription such than when I stop paying the tools stop working. I am getting to the time of life where I am semi retired.  It is not a full time (or even paying)  job for me. So I could be paying for tools for moths when I am not using them.
I don't mind paying Affinity (or Dxo or Black Magic etc) for updates if I need them But if I don't, I don't want the program to stop working (or have to keep paying for things I don't want)

The reason why I want Affinity to do a Lightroom’ catalogue type program is that sooner or later the Lightroom 6.14 will not work with Windows 12,13, 14 etc. Also it will integrate with Photo, Publisher and Designer. 

Which is possibly why we haven't seen a Lightroom /DAM from them so far?
 

I want to pop in here and ask, are people happy with how Affinity Photo handles RAW photos?
My old copy of Lightroom handles RAW pictures a lot better, so much so that I never use Affinity Photo for anything RAW.
Their RAW processing needs to become a lot better if they create a Lightroom competitor.

The good thing about Lightroom is the images stay where they are (and are not touched)  and there is a Library catalogue file.  
Even if the catalogue fails (and the metadata sidecars)  the images and the file structure is unchanged.