It depends on what you mean by forum.
Yeah, a lot of subject-based forums are being "redditified", and other switched to the real-time Slack/Discord model.
Also, I see a lot of community-based products, where forums are just a component (example : outverse [0] ).
But I don't think simple forums platforms are (or never will be) out of vogue. There may even a be lack of diversity right now.
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PS Actually. I always wanted a forum with a UI like Hacker News or Radiohead Message board [1]. I like that bare and outlined design. Does a similar OSS forum exist?