That is in fact what I meant to say, and I thought I said it. Anyway, HTTP/1.1 is still in use a lot of places.

I think most people here don’t have any experience building for the kind of use cases I’m considering here (imagine a proxy like Envoy, which btw does give you at least the option to configure header casing transformations). When you have customers that can’t be forced to behave in a certain way up/down stream, you have to deal with this kind of stuff.

The Go standard library is probably being too opinionated here, but it's in line with the general worse-is-better philosophy behind Go: simplicity of implementation is more important than correctness of interface. In this case, the interface can even be claimed to be correct (according to the spec), but it cannot cover all use-cases.

If my memory serves me right, we did use Traefik at work in the past, and I remember having this issue with some legacy clients, which didn't expect headers to be transformed. Or perhaps the issue was with Envoy (which converts everything to lowercase by default, but does allow a great deal of customization).