Read phpBB-based forums. Many of these 1) are still very active and lively, and 2) include a lot of users who grew up in the 90s/00s internet culture. I like how the phpBB UI encouraged free-form writing/thinking and discussions, with flexible boundaries and etiquette set by the admins or the community itself. Contemporary UIs like Facebook seem far more restricted (or: prescriptive) in this sense, IMO resulting in shorter, kind of impatient and often more aggressive replies. Ramblings on old phpBB boards seem to typically carry more intellectual depth.

Pro tip: to query only phpBB-based discussions via Google, I use the "inurl:" search operator with "viewtopic", which is a standard part of thread urls. E.g. "Windows 2000 inurl:viewtopic".