Use when the user asks to "deprecate this endpoint/field", "set a sunset date", "plan a breaking change rollout", "write a deprecation notice", "how long should we support the old version", or is retiring an API version, a config format, a CLI flag, or an internal library's old interface. Guides the organizational process and timeline for deprecating something already shipped — not the contract-design question itself (see the api-design skill for that).
