Forcing people on community A to go to community B to discuss a subject of A, when it’s perfectly possible that server B is on the opposite side of the world and provides a far woser UX than server A, or is even possible that server B might have defed’d from server A and thus B can not participate, or where the culture of community B is largely different than that of community A (eg.: B treats subject Z as a game; A treats it as a sport) (see also: beehaw vs everywhere else), is honestly one of the most stupidest ideas I’ve heard on the Fediverse. Yes, “most stupidest”, double superlative. That’s how bad it is.
The internet already routes naturally towards guiding people to where content might be. Users on B might link to content on A, at their leisure, but everyone is not forced to lose everything if server A dies or is beehaw. Ideally community members that take part of both A and B can reference both on webring C, because yes webrings are cool and awesome and they should return and they would solve much of this whole issue by raising awareness that A and B deal in subject Z, for the people who care.
And, ultimately, giving the ability to server A to essentially delete communities in server B feels ripe for abouse, and would lead towards a centralization of the Fediverse (exatly what we want to avoid!) simply because sheer statistics means server A sees more use and thus covers more domain space to start new conversations about subject Z, thus pre-emptively deleting them and coopting user activity from B.
Look, honestly: if you want Facebook ot Twitter, go back to them.