I still work on software that extendively uses X- headers.
I wouldn’t worry too much about it. The reason they give is mostly that it is annoying if a X- header suddenly becomes standardized and you end up having to support X-Something and Something. Most likely a non-issue with real custom headers.
The RFC you linked recommends that no new
X-prefixed headers should be used.The paragraph you quoted does not say you should use the
X-prefix, only comments on how it was used.See section 3 for the creation of new parameters: https://datatracker.ietf.org/doc/html/rfc6648#section-3
I wouldn’t worry too much about it. The reason they give is mostly that it is annoying if a
X-header suddenly becomes standardized and you end up having to supportX-SomethingandSomething. Most likely a non-issue with real custom headers.