The spec leaves unsaid how redirections resulting from an (un)subscription POST to a hub are to be treated. Perhaps (un)subscribers SHOULD follow a (small number of) redirections at this point in the protocol? (And hubs SHOULD use codes 307 or 308 to redirect.)

Example use case: sharding based on callback URL. A hub receives a subscription request, determines which hub is responsible for the callback URL supplied in it, and redirects the subscriber to that hub.