#ActivityPub#spam seems to be spreading nowadays. It isn't new, I moved away from my previous instance because it didn't have active mod volunteers.
#Snikket's idea of invites (#XMPP, not AP) seems like a good way to both control spam and onboard users. Mods can rate-limit account creation by judiciously creating invites. Users can generate 1 invite each, but can expect to also get banned if their invitee is a spammer. User-invited users would have a real account to bootstrap their connections.