Some that come to mind to me from the old days (some less old than others):

- "Street fashion" subs, where photos were NOTICEABLY of underage girls taken on the street candidly, usually from a long distance (telephoto lens), and everyone talked about "fashion" in ways you figured out pretty fast were not about fashion. There were a number operating at the same time as a sort of redundancy as they would vanish from time to time.

- CSAM investigation type subs, where users who REALLY knew the lingo and details of CSAM talked about how they were investigating CSAM elsewhere on the internet ... like they were amateur police, but it was kinda indistinguishable from them being connoisseurs. They also would tell each other very suspicious stories about their investigations, like something a jr. high student would make up because they don't know how the world works. It was like they thought they were setting up some sort of plausible deniability or something...

- Gamergate and that whole bizzaro world spanned subs but it was absolutely insane how emotionally charged the devotes were about something as inconsequential as ... video game reviews.