Policy: warn, never silently break.
Deprecated argument names and function names in EventStudy continue to work and emit exactly one deprecation warning per call, pointing the user to the replacement. Deprecated surfaces are never removed in a patch or minor release; removal (the "defunct" stage) is reserved for a future major version and will be announced in NEWS.md.
Mechanism:
Every renamed public argument has an explicit old-name shim that calls
.deprecate_arg() and then forwards the supplied value under the new
name. The shim fires before any computation, so the old name is
fully-equivalent to the new name for all valid inputs.
Warning source:
Deprecation warnings always use base .Deprecated() so
they work without any additional package dependency. When the optional
lifecycle package is installed (Suggests), the warning is also routed
through lifecycle::deprecate_warn() for richer IDE integration;
lifecycle is never a hard Imports dependency.
NEWS discipline:
Every deprecation and rename is documented in NEWS.md under the
release heading in which it was introduced.