Function is_not_supported 

pub fn is_not_supported(status: StatusCode) -> bool

Whether a status is §8.1’s 555.

Worth a name because of what it is not: 555 is not “the server said no” in the way a 403 is. It says the push service the request named cannot be used here, so every retry against this registrar with these parameters will fail the same way. A client that folds it into a generic 5xx retries forever against a plan that cannot work.