It's always struck me as pretty much useless; unless the service is uniquely identifying each downstream and somehow backing them off individually (which is not

It's always struck me as pretty much useless; unless the service is uniquely identifying each downstream and somehow backing them off individually (which is not usually the case, you're usually backing off a consuming identity like an account or an address) then Retry-After is more or less meaning-free for an individual client. In a practical sense, a 429 is just a 503. Back off and try again if you have time, otherwise fail. It's the passive-aggressive "I'm not providing service to your well-formed request, but it's you and not me" response code.