A sunset header tells a client before an email does

RFC 8594 defines a header for announcing that a resource will stop being available, which puts the deprecation in the response rather than in somebody’s inbox.

HTTP/1.1 200 OK
Deprecation: Sat, 01 Oct 2022 00:00:00 GMT
Sunset: Wed, 01 Feb 2023 00:00:00 GMT
Link: <https://docs.example/migrating-to-v2>;
      rel="deprecation"; type="text/html"

// Deprecation: it is deprecated as of this date
// Sunset: it stops working on this date
// Link: where to read about it

Almost no client library surfaces these automatically, which means the header is a record rather than a notification — its value is that it is unambiguous and dated, so a conversation six months later has something to point at. Logging when a client receives one and never changes behaviour is what turns it into a list of who to email.