The Deprecation HTTP response header field is used to signal to consumers of a resource (in the sense of [URI]) that the resource will be or has been deprecated. Additionally, the deprecation link relation can be used to link to a resource that provides additional information about planned or existing deprecation, and possibly ways in which clients can best manage deprecation.
Yeah, sorry. My comment was maybe too curt.
My thoughts are similar to those shared by @Domi in a top comment. If an API user is expected to be wary enough to check for such a header, then they would also be wary enough to check the response of an endpoint dedicated to communicating such deprecation info, or wary enough to notice API requests being redirected to a path indicating deprecation.
I mentioned Zapier or Clearbit as examples of doing it in what I humbly consider the wrong way, but still a way that doesn’t bloat the HTTP standard.