A sunset policy tells clients when you shut down endpoints (e.g. 1 year after a replacement goes live) how users should ensure they hear about this (mailing lists, status pages, HTTP headers, you name it), what they should usually do about it (update, check the docs, follow Link headers). For example, to indicate that an API was deprecated recently, will be turned off in 6 months, link to the documentation, and provide a direct link to the next version, you should include headers like this in the response: Generic HTTP clients can log useful warnings automatically based on this data, API generators themselves can handle more and more of this for you based on API specifications, and HTTP debuggers like HTTP Toolkit can highlight usages of deprecated endpoints for you in intercepted live…