I’ll have to test to clear up the answer to that!
My backend was Nginx which might not specifically be able to do both. I’ll give it a shot with Caddy is the backend.
To clarify: Point 2 above was about the Transport used (I think in the RoundTripper) within the ReverseProxy when it’s taking an incoming request and opening a connection to the backend to proxy to it. The reverse proxy would have to know when it’s h2c vs whatever else, perhaps by making the Transport behave dynamically based on the request.