Ehh. IMO there's still a place for them in extensions that it would make no sense to standardize - this is one example (I hope nobody attaches any behavior to this); a more common one is application-specific headers that aren't generic enough to ever have meaning outside of interop with that one application.

I agree; this is what I thought too, when I learned that "X-" is deprecated, I still thought, that "X-" should still be used (in HTTP, email, Usenet, some file formats, etc) if it has no reason to be standardized. (I have seen other made-up headers in HTTP and Usenet and email, in many cases where there is no reason to be standardized, but a few cases where standardization may be useful, maybe)

You mean deprecated. It also only applies to the creation of new parameters, not to existing ones.