To be fair in some cases rate-limiting could be triggered by a server-side issue, technically there's a 529 error for that, but I can't recall seeing it used.

To be fair in some cases rate-limiting could be triggered by a server-side issue, technically there's a 529 error for that, but I can't recall seeing it used. But 4xx errors should exist as an indication to clients that there's no point immediately retrying the same request. If they're not honouring that, delaying before responding seems the most reasonable course of action.