5×× Server Error
501 Not Implemented
The server does not support the functionality required to fulfill the request.
This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.
A 501 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls1.
501 Code References
Rails HTTP Status Symbol
Go HTTP Status Constant
Symfony HTTP Status Constant