Remove speculation and discouraging language about usage
There's nothing wrong with using HTTP response code 406. This code is the correct and appropriate one to use for its purpose. We should not be discouraging people from using it with this kind of language.
Motivation
The existing language puts a negative framing around HTTP 406 and also makes claims about how widespread the use of this status code is. It is very commonly used in for HTTP APIs and I'm sure it is used at least on occasion for human clients too. Are we able to support this claim? It also says "servers ignore" this header (maybe referring to Accept)? This all seems like pretty strange language to have in MDN.
Steve, thanks much, and congrats on landing your first docs change here — welcome aboard 🎉