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)