Proxies stripping headers isn't a problem if you use HTTPS or HTTP 2. Additionally, the proper place for this is the Content-Type header, which is a standard header any proxy would understand.

I can't say I've come across a framework or a library that makes it impossible to access a non-standard header, and if there are any, that would be a pretty glaring bug. Nevertheless, the proper place for this information is in the Content-Type header, which is a standard header.

In what way is it hard for a human to look at a request to see what's going on? The information is right there.