The Deprecation HTTP response header field is used to signal to consumers of a resource (in the sense of [URI]) that the resource will be or has been deprecated. Additionally, the deprecation link relation can be used to link to a resource that provides additional information about planned or existing deprecation, and possibly ways in which clients can best manage deprecation.
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-Something
andSomething
. Most likely a non-issue with real custom headers.