Oh no! What happened to Guppe groups this time? This points to the one of the biggest problems with all the existing implementations of federated groups/ forums in the fediverse. Like accounts, groups are tied to the originating server, so if that goes dark, they go dark.
I think the Matrix network might be a better place to start federated groups/ forums, at least until the limited data portability in ActivityPub is resolved. At least then groups can persist across all the servers where group members have accounts.
Yes, most of the existing Matrix implementations are chat-orientated, rather than forum orientated, but Cactus Comment is proof-of-concept for uses beyond chat. I’d be intrigued to see someone fork an existing federated forum codebase like Discourse, NodeBB or Lemmy, and change the federation code to use Matrix as well as (or instead of) ActivityPub.