sorry, I had to do a lot of editing in order to get it to post this morning.
Including instances that are also defederated.
Basically, beehaw has decided we can no longer access the "true" version of communities on beehaw. So the versions hosted here on lemmy.world are still visible to lemmy.world users, but that doesn't update the "true" version, and also doesn't update other versions hosted on other defederated instances.
It will be interesting to check beehaw communities hosted on defederated instances in a few days. Because the version on lemmy.world will be very different from the version on sh.itjust.works which will be different still from the "true" version.
So there are whitelist only instances (which honestly is what beehaw should be doing), so if you hosted your own instance, you would need to be whitelisted in order to interact with beehaw communities/users. Otherwise, federation is pretty much a default
Ok, so this requires some understanding of the ActivityPub protocol, and my understand of this edge case is admittedly a bit fuzzy. You can still access that information, you could do it right now just by going to https://beehaw.org, and if you have some mechanism to pull that data, you could still get that data if you wanted to. But critically, that wouldn't use ActivityPub.
With ActivityPub, your instance would send a request to the community on beehaw to follow the community. The beehaw instance would then send updates to your instance, where they would be stored as a copy. Beehaw keeps the "true" version, as the community is hosted on their instance, but you have your own copy. If beehaw defederates you (or is whitelist only and never federates you), then you can't send that request (rather, you can send the request, but beehaw won't listen). So beehaw will not send updates via ActivityPub.