The CONNECT-QUIC method as defined in this document can only be supported by an HTTP/3 proxy. Use of CONNECT-QUIC with older HTTP versions is undefined and MUST be rejected.
But RFC7231 says:
When a request method is received
that is unrecognized or not implemented by an origin server, the
origin server SHOULD respond with the 501 (Not Implemented) status
code.
The spec says:
But RFC7231 says:
Just checking the two statements are compatible.