Really though? You can implement the same limits for federated posts, and just drop the ones exceeding the rate limit. Who knows, might be frustrating for normal users that genuinely exceed the rate limits, because their stuff won't be seen by everyone without any notice, but if they are sane it should be minimal.
The notice might still be able to be implemented though. idk how federation works exactly, but when a federated post is sent/retrieved, you can also exchange that it has been rejected. The local server of the user can then inform the user that their content has been rejected by other servers.
There are solutions for a lot of things, it just takes the time to think about & implement them, which is incredibly limited.