There's nothing comparable for forums. FB tried but it's a cesspool of "user engagement", managing groups is a full time job and still doesn't work properly.
Oh, and Discord _has_ a forum-like feature inside it :D
Matrix is its own brand of confusing because it's trying SO hard not to be a Discord clone. If you properly want to own your Matrix channels, you need to run your own server and that's not a 3 minute thing to do.
Pretty sure the last bit in the GP was the important one:
> What happened to product forums _that could be easily found in a web search_?
Without web searching capability, whatever "forum-like feature" exists is still subpar.
Content is either in silos like FB or Discord or in videos on Youtube. None of which is properly searchable from an external search engine by content.
The only negative feedback I've gotten is from users in China, where Discord is blocked.