It seems to me that beehaw could already accomplish something along these lines with an "automod" equivalent that just insta-rejects any outsider posts on their hosted communities. So having that built into the protocol as a flag of some sort may actually be a good idea, because then other instances would be able to more easily see that behaviour and choose to not federate with them in return. If I was running a Lemmy or Kbin instance I wouldn't want to bother my users with communities that they could see but not interact with.