501 Not Implemented
A server can return 501 Not Implemented when it doesn’t support a
certain feature.
The RFC specifically lists ‘not supporting a specific HTTP method on any resource’ as an example of this.
In most practical cases this is similar enough to 405 Method Not Allowed,
and 405 is probably the clearer status code for most cases.
But there is a difference. Given that 5xx are server-side errors and 4xx errors are client-side errors, it could be argued that emitting a 405 means that a method should never really have been called by a client, but a 501 could mean that the method should have worked, but the server has incomplete functionality.
I sometimes use 501 during development of new API’s as a ‘stub response’ for features I intend to build, but haven’t built yet.
Example
HTTP/1.1 501 Not Implemented
Content-Type text/html
<img src="http://geocities.example/work-in-progress.gif" />
References
- RFC7231, Section 6.6.2 - 501 Not Implemented