Declared by three providers in the entire catalog. The market's answer to "when does this endpoint die" is a blog post.
Deprecation
Versioning & Lifecycle IANA permanent response reached by regulation 2 spellings
Deprecation (RFC 9745) tells a caller that a resource is on its way out, and pairs with Sunset (RFC 8594), which says when it goes away. Together they are the machine-readable answer to the question every integration eventually asks.
Across 26,641 providers and 119,030 published specification files, this header is declared by three. Sunset by six.
That is not a gap in the measurement, it is the finding. The entire API economy communicates breaking change through changelogs, emails to an address someone left the company four years ago, and blog posts. None of that is reachable by the software doing the integrating. An agent calling your API has no way to learn that it is calling something scheduled for removal, and it will find out the same way everyone else does — when it breaks.
The registry
Listed in the IANA HTTP Field Name Registry as a permanent entry. Defined in RFC 9745, Section 2: The Deprecation HTTP Response Header Field. It is a structured field of type Item (RFC 9651).
In the catalog
Declared by 3 providers across 652 published specification files in the API Evangelist catalog, where it appears as a response header — sent by the server.
It is spelled 2 different ways across those contracts — deprecation, Deprecation. HTTP field names are case-insensitive (RFC 9110, §5.1), so every one of these is the same header. They are not the same string, which is why generated clients disagree about it.
Reached by regulation
No law names this header. It is evidentiary — the deployed control for an obligation that regulation does impose. It is visible in a published OpenAPI or AsyncAPI, so the catalog can count it.
Using it
Send Deprecation as soon as the decision is made, not when the shutdown is imminent — the value is a date, and a date in the past is legitimate and means “already deprecated”. Pair it with Sunset for the removal date and a Link with rel="deprecation" pointing at the migration guide.
Declare all of it in your OpenAPI, in components.headers. A header that only exists at runtime cannot be discovered by anyone planning an integration, which defeats the purpose.
Reached by these regulations
Catalogued at regulations.apievangelist.com, with the basis of each connection recorded rather than implied.
Governed by these rules
Machine-enforceable governance rules from rules.apievangelist.com that apply to this header when it appears in an OpenAPI.
OpenAPI Components Headers Error error
Utilizing the headers object in the centralized OpenAPI components library helps make headers reusable across API requests and responses
Guidance: Rate Limits →OpenAPI Components Headers Info info
Utilizing the headers object in the centralized OpenAPI components library helps make headers reusable across API requests and responses
Guidance: Rate Limits →OpenAPI Headers Hyphenated Pascal Case error
HTTP headers should follow Hyphenated-Pascal-Case naming convention for consistency and readability, such as Content-Type, X-Request-Id, or Accept-Language.
Guidance: Naming →