Frameworks like WCF (Windows Communication Foundation) used to support different transport level protocols, like TCP, but IMO that discussion is long over, with

Frameworks like WCF (Windows Communication Foundation) used to support different transport level protocols, like TCP, but IMO that discussion is long over, with HTTP the obvious winner. And of course, the overhead is even less these days with HTTP/2. I find GraphQL not being coupled to HTTP to be a problem, no a benefit - as you say, returning HTTP 200 for everything is just daft.