The hard part is that there is no good way to confirm whether an account or a community is truly abandoned. Some users prefer to lurk. Some users don't reply to Lemmy private messages. Some users keep an alt account for moderating and use another account as their daily driver, so the mod account looks inactive even though there's an active user behind it. Some users are sitting on an empty community with aspirations of growing it in the future when they get some time. 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.