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.
few communities exist online in only one place anymore (for example, lemmy itself has a Matrix space, a #hashtag, etc). i think forums, IM, and the different kinds of (micro-)blogging coexist to encourage participants to balance length and substance/effort to different degrees in their discussions. IM favors short, distilled ideas; blogs favor lengthy in-depth things but don’t tend to encourage as much direct participation, and forums fill a middle ground.
but forums are also a victim of their own success: a highly active thread means you have to read for an hour before participating, or else you contribute something that’s already been said and you just add to the unwieldiness! tree-style comments really do let people selectively explore different, narrow slices of a topic without creating that mess. it’s not perfect, but if i’m forced to choose one or the other, it’s usually an easy call to make.
on the other hand, you could argue that some portion of “threads” in a chat app (particularly ones that live for more than a day) are really just watered down forums. so maybe we’re destined to recreate these things without realizing it, just in ways that borrow from all these different modes of communication in less rigid ways.