First off, this is the wrong sub for this question. See the sidebar, but this is a kind of random discussion sub for lemmings, similar to /r/AskReddit/. The Lemmy or lemmysupport communities would be better locations, as would the meta community on lemmy.world... which I s called lemmy.world (and unfortunately has lots of off-topic junk in it right now). But your question is an important one, so I'll weigh in here.
I'm not 100% certain, but I expect these zombie communities are unmoderated. The right thing to do would be for reports to go to instance-admins, but I'd be surprised if Lemmy covered this edge case correctly right now. I think there's a decent chance you may have to at-mention or DM an instance admin to get posts/comments in these orphaned communities addressed, and also that admins on affected servers should consider purging them to address this issue of lack of moderation.
This is one more example of how this split increases moderation load network-wide, and how Beehaw is offloading the costs of their unsustainable moderation policies to others.