I don't know how much it affects priority, but they certainly accept bugs filed against older versions. Specifying versions is a big part of the form for a new bug, and they let you select versions going way back.
Which is a problem. NixOS remains by far the least aggravating OS I know, and... yeah. I wish there existed a desktop environment that played well with it.
But also: Isn't one of the main benefits of NixOS that you can fairly painlessly get and try different versions of things? Just see if you can reproduce the bug in the same version of upstream. If so, file the bug with kde, if not, file the bug with NixOS because it's presumably caused by one of the patches.
That can be up to and including a downloadable VM image that shows the issue.
If you have a specific issue under nixos, it's going to be fairly easy to see if it's very unlikely to be caused by custom patches.
For reference I count about ~66 patch files among the KDE expression in nixpkgs as of today, including Plasma, all libraries, and related apps. Most of them are in the range of 20 lines long, and they are .patch files, i.e. the actual applied diff is smaller than that. The largest patch is barely 190 lines long and it's for Akonadi, mostly rewriting hardcoded FHS paths throughout the codebase.
I agree there are some quirks with most desktop environments on NixOS in my experience but realistically there's a huge amount of stuff in the ecosystem that plays anywhere from well-to-poorly in such environments, and the Linux desktop stack definitely was not designed at a time where this stuff was common. It is what it is, I guess.
This is how it used to work (and still does with enterprise distros, because you might as well use the support contract you paid for). But users these days have gotten savvy enough to start engaging with upstream directly, especially since many upstreams have made it easier to be engaged with by using GitHub etc instead of mailing lists etc.
Sometimes engaging directly with upstream works, as it apparently does with KDE according to other comments in this thread. Sometimes upstream gets annoyed because they only care about their tree, eg systemd devs get annoyed when users report bugs that have long been fixed in master, but still exist in an old stable release that happens to be the latest on some LTS distro. It depends on the project, so when in doubt start with the distro tracker.