501 Not Implemented
This is an educational reference page about HTTP 501 Not Implemented. The page itself is served as 200 OK so it can be indexed as HTTP documentation.
The server either does not recognize the request method, or it lacks the ability to fulfill the request. Usually this implies future availability (e.g., a new feature of a web-service API).
Specification
Registered in the IANA HTTP Status Code Registry as Not Implemented. Defined in RFC 9110, Section 15.6.2.
Header and content rules
RFC 9110 adds no header or content rule specific to 501. The 5xx class rules also apply. See 501 in the response obligations table.