When I eventually start working on Shortwave (my Common Lisp Fedi server project) again, I really want to implement a user-whitelist mode for remote instances.

Essentially, it would block all federation traffic to and from the restricted instance, except for what should be visible to a set of known-good users (either manually approved by an admin, or followed by local users). All posts from local users would appear as followers-only to that instance, and wouldn't even reach it unless the author follows an account on there.

Also, Shortwave could potentially exchange block information with other servers, to prevent remote replies and shares from reaching users or instances the author (or author's instance) blocks. This would probably be done with a Bloom filter, so remote instances can't easily determine your exact block list. But this would only work on instances with explicit Shortwave compatibility.