> In addition to offering human-understandable communication, I asked the API producer to add the Deprecation HTTP header field to all responses
Cute, but, I question the value
Cute, but, I question the value
I came here just to say that. What an half-baked idea. It might be trivial to mindlessly bolt on response headers, but if the goal is that the mechanism needs to be impactful and have consequences then the response header is a big red herring as you're actually relying on clients to implement support for sunsetting the endpoints. If that's the case then you already have meaningful mechanisms, such as passing this sort of metadata in responses to requests for the root resource. This is something that pretty much any HATEOAS spec already supports.
Yes, they support it with headers.
As a consumer, you can set up an alert when any of your API requests has a deprecation or sunset HTTP header. You'd know immediately if any of the APIs you depend on is about to be deactivated.
If they were competent enough to do that, they'd probably pay attention to updates sent via, say, email. Brownouts would make people notice, like "oh shit, $API doesn't work!"