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 existi...
Bloating HTTP and its implementations for REST-specific use-cases
I have no idea what are you talking about. Setting a request/response header is not bloating HTTP. That's like claiming that setting a field in a response body is bloating JSON.
Proper HTTP implementations in proper languages utilize header-name enums for strict checking/matching, and for performance by e.g. skipping unnecessary string allocations, not keeping known strings around, ..etc. Every standard header name will have to added as a variant to such enums, and its string representation as a constant/static.
Not sure how you thought that shares equivalency with random JSON field names.
Proper HTTP implementations in proper languages utilize header-name enums for strict checking/matching (...)
I don't know what you are talking about.
Java provides java.lang.Object.HttpHeaders, which is a constants class that provides static final String fields for the popular request and response headers.
.NET does the exact same thing with it's class Microsoft.Net.Http.Headers.HeaderNames.
You just referenced two languages that don’t have proper sum types. lol.
You're complained about "Proper HTTP implementations in proper languages".
I provided two concrete examples of two of the most popular and production-grade programming language ever developed.
I can provide more.
You then tried to weasel out by moving your goal post from "Proper HTTP implementations in proper languages" to "languages that don't have proper sum types".
I won't waste more of my time with you. Whatever you're posting lacks relevance and does not justify any attention from anyone.
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.