And add deprecation notice.
Updates golang/go#60951

Change-Id: I9d4ffffc03201b038db5cc11563ea1cc5330e668
Reviewed-on: https://go-review.googlesource.com/c/tools/+/563398
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>