Just saw this is out.
I suppose people will expect it all to be implemented/supported when GIMP 3.2 is out (and we should).
Assuming libpng is updated on all Linux distros at the same time…
We can certainly speed that up by setting the minimal required libpng version.
But even if 3.2 is done in a timeframe too short to allow libpng cover these new specs and spreadout, there is work on GIMP’s side which is done outside of libpng, isn’t there? Checking this code is conforming to the new specs, as far as possible is something we can do - and AFAIK, there is at least work being put APNG which is now specced out along with PNG in this doc.
(Also, I believe a fair share o GIMP is distributed with needed libraries static-linked these days, so we don’t have to wait for some debian-classic-slow-fork to catch up with the needed libpng)
Funny enough, we just added support for importing APNGs in GIMP 3.1.2, though via working around libpng’s limitations. 
There’s a pending branch for this support in libpng, so once that’s released we can start implementing the new API. The WIP libpng code has a macro for APNG, so we should be able to section that off when building.