RFC 822 makes a promise that fields beginning with X- will never be
standardized. If RFC 822 is truly obsoleted by a document that
doesn't restate that promise, then the promise will have been lost;
"obsoleted" is not a formal action, it's just a label put on by the RFC
Editor. and as in this case, it is sometimes misleading.
My view is that the text in RFC 822 stands until it is formally and
explicitly changed.