It works on the individual instance, which for all intents and purposes means it works for everyone since everyone can see the instance.

You just have to make sure you're pinging the right admin for your instance. In this case it's ruud I believe for lemmy.world

This is my understanding - federated sites push out their content, so just like a new post will propagate into any place it is subscribed, a deleted community/magazine will also propagate (and I suppose remove the subscriptions? Just as housekeeping). I know I've seen that is how removal of prohibited topics would work, so it should be the same.

This shouldn't be done arbitrarily, sure, but there should be some rules in place for these things. Like, two weeks of unresponsiveness by the mods for example.

So whats to stop it being filled up with garbage ?