For me internet is built on one simple principle: links. One forum should just agree to discuss this subject on another discourse instance -> “Closed topic: follow discussion at http://xxx” If they can’t agree on this I don’t see them agree on any of the things I will explain later.
This proposal is raising tons of questions which won’t have any easy answer. What if different communities have different moderation rules (who decides if this post should be deleted?) ? what if one user is banned on one forum and not the other? What if both forums don’t update at the same time, it could be a nightmare for us to handle those cases… And I’m not talking about all the technical questions it raises as they could ultimately probably be solved but with much complexity.
I can see some value in the proposal, but as I see it, simply relying on internet principles and keeping it simple is a much better approach.