It should be noted that nearly the same verbiage presented by the GP is used in RFC 9110. RFC 9110 also goes further to explicitly say that 999 is an invalid er

It should be noted that nearly the same verbiage presented by the GP is used in RFC 9110. RFC 9110 also goes further to explicitly say that 999 is an invalid error code, but does indicate they should be treated as a 5xx. > Values outside the range 100..599 are invalid. Implementations often use three-digit integer values outside of that range (i.e., 600..999) for internal communication of non-HTTP status (e.g., library errors). A client that receives a response with an invalid status code SHOULD process the response as if it had a 5xx (Server Error) status code.