The 501 Not Implemented status code represents a fundamental aspect of HTTP method and feature support. Implementation requires careful consideration of API versioning, feature detection, and client communication. The response should clearly indicate what functionality is not supported and why. Feature detection mechanisms should be comprehensive, potentially including OPTIONS method support for discovering available methods. API versioning strategies must consider how to handle requests for unsupported features across different versions. Security implications include protecting against probing for unsupported features while providing appropriate error details. The 501 status code serves as a key tool in API evolution, requiring thoughtful implementation to maintain clarity and security.
