There seems to be a long time before 6.0 is good enough to be released and there are bugs fixed there that people running 5.12 are affected by so what about taking the known fixes and showing them into a 5.13 release?
It would be nice if there was a “long term support” - version of ardour that gets nothing but bug fixes and a separate development version that gets the new features.
I known this would be more work, so it may not be possible for the small development team, but one can always hope 
That’s not going to happen. The changes in the current master are deep and wide-ranging. We haven’t even attempted a “bug fix branch”, because we haven’t actually had a focus on bug fixes. A few have happened as a side-effect of other work and perhaps 2 or 3 have actually been done intentionally. But we don’t have the resources to put into a bug-fix release process, and frankly the difference between a “5.13” and the current 5.12 would be tiny.
Is there something specifically that you know is in master that you think should be released?
Nothing specific, Paul, other than I’d imagine quite a few bugs in the 5.12 codebase must have surfaced in the 9 months since the 5.12 release and I saw that the Korg nanoKONTROL issue was one of them.
https://community.ardour.org/node/15708
I do realize the Ardour team is a bit on the small side and that doing a big overhaul on the codebase while at the same time releasing bugfixes to the old one might be too big a challenge.
Was that the reason for not developing v6 in its own branch, leaving master as the v5, bugfix only branch; because there’s no one with the time and energy to maintain that?
v6 is developing in its own branches.
we’ve just already merged a couple of them back to master. our development process does not reserve master as “the last release plus bugfixes”, though of course I’m aware of some projects which do. we rely on many testers who use the nightly build, which is expensive enough (it takes 5-6 hours to do the linux and windows builds alone) that we only build one branch. that branch is … you guessed it … master. so at all time, master is stuff we appreciate feedback from our alpha/beta testers. it is not as close to the last release as possible. we have never opened a “hotfix” branch, in fact - when those things have been needed, it has been very soon after a release, and we just do another one.