Function retryable_status 

pub const fn retryable_status(status: Option<u16>) -> bool

The one status → retryable table.

Request Timeout (408), Too Early (425), Too Many Requests (429) and every server error (5xx) are retryable; every other status is not. A missing status decides nothing here: a failure with no status is classified by what it is (transient_transport), not by what it lacks.