If the client does not receive this header in responses, the client SHALL assume that the proxy
does not understand how to parse Connection ID capsules, and MUST NOT send any
Connection ID capsules.
Followed by
Note that the initial REGISTER_CLIENT_CID capsule MAY be sent prior to receiving an
HTTP response from the proxy.
Clients wishing to send capsules along with request headers cannot do so without violating the requirement that the server responded with the Proxy-QUIC-Forwarding header. Since CONNECT-UDP implementations MUST be able to parse capsules (and ignore capsules they do not understand), I think it's safe to remove the "MUST NOT send any Connection ID capsules".
This seems contradictory
Followed by
Clients wishing to send capsules along with request headers cannot do so without violating the requirement that the server responded with the Proxy-QUIC-Forwarding header. Since CONNECT-UDP implementations MUST be able to parse capsules (and ignore capsules they do not understand), I think it's safe to remove the "MUST NOT send any Connection ID capsules".