I think NNTP+IRC is good, and you can have bridging to other services if desired (which is probably a good idea too). You should also make logs of the IRC; the

I think NNTP+IRC is good, and you can have bridging to other services if desired (which is probably a good idea too). You should also make logs of the IRC; the IRC channel for my project has public logs. Discourse and Flarum are even worse, in my experience. However, having chat services does not substitute for having documentation; you should also have good documentation, too; you should not expect someone to only ask questions.