@aral There's two test cases I'm thinking about right now:
in 1997, I ran a wiki for unschooled teenagers. It eventually bit-rotted, and the reins never got handed off properly. (the technical barriers were too high), but but it was collectively managed for the most part. That was super successful until it aged. It was such a solid sense of community, and people often posted pseudonymously, there were no accounts (very 1997 there...), spam was collectively erased. There was a lot of good there. Trying to replicate anything like that today would be really hard, both for external background toxicity reasons, but also technical shape of things is really different now.
And the other is our local "Salem Safe Walking Advisory Group" — right now, the infrastructure is owned by the person who started the group, and if anything ever happens to her, the group will cease to exist. But it's not something that would get sustaining power from being embedded in my and the other organizers social media space. It's something that demands an identity of its own, shared.