I think single-fandom forums have been replaced by fandom-specific Discord servers in terms of their function in the fandom ecosystem, actually! They serve as spaces where internet users with nothing in common but their fandom get together to discuss their interest in even more specific fandoms.
Although Discord servers have their own differences and downfalls compared to forums: Private instead of publicly perusable by lurkers, the whole over/under 18 nsfw channel Stuff, linkrot when the Tumblr and Twitter posts people share in them get deleted or their URLs changed, 2000 character limit for messages, etc etc, there is no expectation for a Discord server to be pan-fandom. I've even seen servers dedicated to single rare ships in the sports anime fandoms I'm in.