A deprecation is a dated promise: it names its replacement and the version the old name stops existing in. GemKit::Deprecate declares that promise -- `deprecate` for a method, `superseded_by` for a renamed constant -- warns on use naming the caller, and registers the deadline so it can be checked. Built on Gem::Deprecate, so the message format and Gem::Deprecate.skip_during work as they already do. No dependencies beyond RubyGems' own: a library that deprecates a name should not thereby acquire a release toolchain. The checking lives in the gem_kit-release gem, which reads this registry.