The hard part is that there is no good way to confirm whether an account or a community is truly abandoned. Any criteria for determining what gets removed is going to have a significant false-positive rate and piss off a lot of people.

I'm not sure there's much to gain from a mass removal, anyway. Lemmy.world has several thousand communities, most of which are pretty quiet, but the mere existence of those communities isn't causing any issues as far as I know.