I think the goal was for SEMANTICS to stay out of message framing and leave that to the individual mappings. It currently says this about CONNECT:

Any 2xx (Successful) response indicates that the sender (and all inbound proxies) will switch to tunnel mode immediately after the blank line that concludes the successful response's header section; data received after that blank line is from the server identified by the request target.

Perhaps this should simply talk about data received after the header section, and leave the boundaries of that version-specific?