Also, TIL that the IETF deprecated the X- prefix more than 10 years ago. Seems like that one didn’t pan out.

Can you elaborate on that? The X- prefix is supposedly only a recommendation, and intended to be used in non-standard, custom, ah-hoc request headers to avoid naming conflicts.

Taken from https://datatracker.ietf.org/doc/html/rfc6648

In short, although in theory the "X-" convention was a good way to avoid collisions (and attendant interoperability problems) between standardized parameters and unstandardized parameters, in practice the benefits have been outweighed by the costs associated with the leakage of unstandardized parameters into the standards space.

I still work on software that extendively uses X- headers.