this is an interesting question i’ve had banging around in my mind since well before Reddit’s implosion (and Discord’s enshittification), but which seems really worth asking now.
you can’t blame Reddit and Discord or their imitators entirely for these going out of style, but they’ve sure put the dagger in a lot of remaining ones, and i kind of wonder if they’re just in an irreversible and terminal decline a la USENET. i can only name two or three i even consider checking anymore, and i’m not sure how sustainable any of those are long-term.
There certainly seem to be topics I track where forum activity outpaces Reddit. Haven’t explored Discord for the purpose. I would like to see communities emerge or move to something on the activitypub - lemmy devs made a forum frontend (https://github.com/LemmyNet/lemmyBB). The potential seems to be there to provide a forum interface/experience that federates.