Private torrent trackers have been doing this for a while. If some number of your downstreams act like shitheads - you get nipped and so do your other downstreams.

This seems like the best way to handle it. Also, smaller communities. It's cool to do the global thing, but once you have 10k active users you can't moderate it with a team of 5 volunteers.

I think the attestation approach works best if there are different reasons for the punishment. Eg someone inviting a turd doesn't ban the person who invited them. Someone going full ai spam should.

Was it demonoid? That was like this way back in the day? Needed an invite and if you leeched you were cut.