Largely the same as if they just disappeared and stopped logging in. The community would become abandoned and unmoderated.
Reddit had a system where you could request banned or inactive communities, Lemmy instances don't usually have this, instead requiring you message that instance's admins directly.
I mean really any instance can do it, it's as simple as creating a community to request other communities. Most just haven't chosen to do it, unfortunately.
That's true. kbin's was a bit more slick - you could click a button that was on the page for the magazine (community) itself to make the request, but really it's just a minor UX convenience
That is actually a pretty nice feature. Would be great if we had it here. Though it isn't explicitly needed, request communities are also a good solution.