https://github.com/LemmyNet/lemmy
I know of the dev instance and chapo.chat, but I'm sure there are a few others.
I don't see why it wouldn't work. Matrix is a solid protocol to work with. The only thing I want to be careful about is ensuring that the UI has none of the weirdness usually found in decentralized apps.
As a discussion system, it's pretty much defunct at this point.
It's pretty much fine as a discussion forum.
Things have been lively in comp.lang.c recently with a little coding exercise going on and such.
comp.unix.programmer and comp.unix.shell are active also.
comp.theory has been quite active due to a crackpot who insists that he has solved the halting problem, or at least refuted common halting proofs using a nested x86 simulator that detects recursion, haha.
....just went down the rabbit hole.
There is lots of bbs stuff out there. Here is one such example: https://github.com/NuSkooler/enigma-bbs