In a final push to close the open issues about adding more Web Concepts, HTTP Preferences (established by RFC 7240) and HTTP Forwarded Parameters (established by RFC 7239) are now part of

Interestingly, both registries have no entries other than the ones established by the registry-defining specifications themselves. This often is a sign that a registry may not have been necessary, but that is sometimes hard to predict, and of course future specifications might very well add new values to those registries. For now, I would be interested in feedback if anybody has seen non-registered values being used for those parameters in the wild.