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.
We don’t have many unit tests that test against live APIs, most use mock APIs for testing.
The only use for this header would be if somebody sees it during development, at which point it would already be in the documentation or if you explicitly add a feature to look if the header is present. Which I don’t see happening any time soon since we get mailed about deprecations as well.