Apfel

I was just looking for this feature and was surprised to find that it doesn't exist... so you've got another vote for a free transform feature.

I'm aware that it doesn't/shouldn't work, the problem is that the app gets stuck, which shouldn't happen no matter what.

I would also like to have that feature. If you add a Symbol to a document and don't want it to sync, you can simply unlink it. It you use lots of global symbols and can't keep track of where they are used, it's a) not much different than having a single document with lots of artboards, and b) it's the users responsibility to not make things overly complicated.
One way to implement this, by the way, would be to allow "Place Image" to link an image instead of embedding it, and refreshing whenever the source file changes. Then, your global symbol would be just a document that contains that symbol only.

Thank you for all the answers! Maybe I should have added some context on what I'm actually trying to do, but it's slightly complicated.
So I have a background pattern that consists of a lot of lines. I want to "cut out" an area of that background pattern, e.g. imagine I want to fill a circle with the pattern.
Initially, I used a mask for this, but then I found out that Affinity Designer then rasterizes the pattern when exporting! It won't rasterize when I remove the mask, so I had to find a different solution.
My solution was to take all the lines, expand the strokes, union the resulting curves, and then use the intersect tool to restrict the pattern to the shape I want.
I was unhappy with this solution, because I wanted to achieve this without expanding the strokes.
 
@firstdefence: The snapping feature of the node tool makes this a lot more doable manually, but I would have had to cut hundreds of lines in my case, which made that approach unusable. I learned something about the node tool though!
@Alfred: Wow, I totally didn't realise that just adding a curve to another curve makes the parent pretty much work as a mask! Affinity will export it without rasterizing, too. This is awesome, I definitely learned something new here; this approach works to solve my original problem. If someone else has the same problem, here would be my adjusted instructions:
Create a rectangle the size of your document, set fill and stroke to transparent Subtract the area you want to hide Follow @Alfreds step 4 and 5 @JimmyJack: Yes, because the gray rectangle is just to specify the area I wanted to cut. I do not want to have it in the final design - the area of the rectangle needs to be transparent in the end.
 
To summarise: Thanks, you found a solution to my original problem!
However, I would still like to have a tool that allows me to cut lines using another line or shape. Maybe I should create a feature request, because it looks like it doesn't exist. I now know how to work around it in certain cases, but it's not too hard to imagine a scenario where the approach from above won't do. For what it's worth, in Inkscape, the tool I was hoping to find is called "Cut Path".