I've come back to this topic after three months to see if there has been any progress.
There has! The roadmap has been removed from the public facing internet.
So it seems like any pretence this was going to get fixed after 5 years on the list has been dropped.
As I think we've established in this thread, pretty much all competent web designers require this feature for image manipulation software to be useful. I do think that if you are saving out images without bothering to care about what they finally look like, then you are wasting your time using complex image manipulation software. 'Paint' is there for you, bro.
So it's back to the tender mercies of Adobe. Such a pity, I was really looking forward to giving the young upstart a chance.