Maybe I'm just missing it in the docs, but I've also tried to find it in the code, and I can't seem to find anything there either:
Is there a way to tell traefik that your backend server supports HTTP/3 and that you would prefer that it connect to your backend over HTTP/3? You can set up the server transport with the correct certificates to use HTTP/2 and TLS on the backend. I would expect the option to use HTTP/3 on the backend would be there as well, but I didn't find it. Maybe it isn't supported? It looks like right now Traefik can accept HTTP/3 connections, but can only forward them as HTTP/2 connections to the backend. Is that correct?
I tried setting an Alt-Svc header on my backend, but Traefik is just passing that through to the client, which isn't what I expected at all. I was trying to tell Traefik how to connect via HTTP/3. The ports are all wrong to get passed to the client.