The #ActivityPub sharedinbox is a strange concept. It's an inbox without an actor. I suppose one could think of it as being backed by an implicit, non-conformant (no outbox) actor-ish thing that routes messages to other inboxes. There's no requirement that there's zero or one sharedInbox per server. A server can have any number of them. You could have one per set of actors in a Group or Organization, for example. But then the sharedInbox would an Group/Org actor-specific inbox. 🤯