the thing "crappy" forums had that modern platforms killed: you were talking to the same ~200 regulars, not performing for an algorithm — small and stable beat big and optimized.
This is a great point, having experienced just this firsthand just recently. I launched an online boardgame and tried announcing it in a bunch of different places - boardgamegeek, some relevant subreddits, elixirforum. While the other forums got views, the one that actually got some real engagement and comments was the boardgamegeek forum, arguably the "crappy" one of the three. Although it's "crappy" only in the sense that it's pretty no-frills and just feels kind of old-school.
exactly — "no-frills" and "old-school" usually just means the UI isn't competing with the conversation for your attention.
Discord and IRC feel like this
Interestingly IRC used to have, but no longer has, a feeling that you are writing for people who are online right now, because message delivery was only real-time. It still is, but now everyone uses bouncers or always-on connections so people who return hours in the future are more likely to reply to what you said.