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.
It makes sense to include so it’s obvious in the readable HTTP request response. We use readable URLs and header names for the same purpose: So it is inspectable and understandable from that text format. You may leave deprecation information out, but then you’re missing part of the resource description that you’re addressing with the URL/URI.
Given a defined header it also allows you to add tooling and automation. There’s no need for manual reading.