The interesting thing is that in fact you can still post stuff to lemmy.world and sh.itjust.works or any other now de-federated communities so long as it was syncing before.
HOWEVER, only beehaw members can see those new posts, and posts and comments from other instances cannot be seen.
Example: on Lemmy.world's no stupid questions community, someone has a post on the cost of games, that's a beehaw only post.