I find that if a POST fails to be processed I don’t really want the sender to retry anyway, I want them to stop doing it. So if the sender thinks it was successful it’s usually not the worst thing in the world.
It would be nice if Nginx responded with a HTTP 202 (Accepted, yet queued) if a POST was throttled and it would be nice if sending fediverse software knew what to do with that info. But I expect this is an edge case that hasn’t been dealt with by most.