I don't really get the purpose of a header like this, who is supposed to check it? It's not like developers casually check the headers returned by an API every week.
Write them a mail if you see deprecated functions being used by a certain API key, probably much more likely to reach somebody that way.
Also, TIL that the IETF deprecated the X-
prefix more than 10 years ago. Seems like that one didn't pan out.