There's really no problem with X-* at all; the rules in 822 are clear.
Then what is the problem including those rules in a revised document?
the problem is that it's a political can of worms. there were people arguing
that X- should be legitmized, and there were people arguing that legitimizing
X- would destroy the entire purpose of X-.
If a revision of 2822 is moved to Draft Standard, and fully obsolete
822, I don't believe that what 822 says or not matters.
822 is going to be definitive for as long as people want their email readers
to be able to read messages that were created before 2822 (or its successor)
became (a) full standard and (b) widely deployed.
It would be
sufficient that the revised document acknowledge that X-* are not
discussed, if the perception is that nothing else can be agreed on.
I don't think that belongs in a standards-track document. A separate document,
perhaps.
Acting as if X-* never existed is not enlightening.
nobody is proposing to delete all copies of 822 from the archives.