LPT: The Lemmy-Federate project is able to do that automatically with a single click by using a bot on each participating server, which then unsubs once a real user from that instance subs to the community. 😄
Ah, that makes sense. It'd still be nice for it to be baked-in functionality that's just disabled by default. A bot that pretends to be users seems unnecessarily hacky, but ultimately it probably doesn't really matter much.