Lemmy.world users can still post on beehaw communities but only other lemmy.world users will be able to see and interact with those posts. You’re probably seeing people post on the same beehaw communities that they subbed to either without realizing that they are defederates or not understanding what that means. It will basically always be better now to find somewhere else to post if you want to reach a wider audience now. So instead of using the beehaw gaming community it would be better to use a lemmy.world one for example. That being said, since lemmy.world is the largest instance at the moment, even posting on a beehaw community and only reaching other lemmy.world users who happen to be subscribed to that defederated beehaw community (or who browse all) will still probably reach a wider audience than someone in beehaw posting to other beehaw users for example.