503 could still be useful if combined with the Retry-After header but I wonder if any generic client code/library uses it in that way.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Re...
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Re...
Also, isn't HTTP 503 just about the same as 500 as far as your garden variety client is concerned--even if it's technically plausible some client, somewhere, might want to more aggressively seek out a stale cached result for a 503?