Forums got replaced by Facebook for the most part, and the ones that didn't gradually changed as adherents grew older and normie standards started taking over the Internet in earnest.
By way of example, I used to be a fairly well known member of a very tiny Playstation gaming community that started in 2000. At our peak our forums had about 1500 users, with 2 or 300 online at any given time. Everybody knew everybody, it was comfy, we talked about everything and even did our own tournaments, some of us flying cross-country to attend.
But by the time 2008 rolled around, the community was dying. Everyone had migrated to Facebook for their daily dose of Internet chat, and cliques started to form amongst those left. One of our members was going to college in California and turned into a complete fruitbat - what you'd call an SJW today. He starting bringing social justice and political discussioin in, hot and heavy, and the board fractured along political affiliations. Conservatives were in the minority and it showed: "Liberal" users could get away with murder, but if you so much as raised your voice while arguing with one, you'd get a warning or a ban. One of the admins permaderezzed me in about 2010 for posting a meme he found offensive and I never posted again. I check up on the old place about twice a year, and the only people left are a half a dozen of the old guard who sound, without exaggeration, like NeoGAF.
I left that community and became a regular in another nice spot. We had a lot of International peeps and some really good discussion took place. But it came tumbling down a couple years later when they started an IRC. A clique formed in the IRC chat, consisting of the admins, mods, and a group of popular users. The whole tone of the place changed, where shit-talk, egos, and personal drama got carried over from the IRC and onto the forums, and people outside the clique were made to feel unwelcome. That site died entirely in about 2015.
Feels bad, man.