Deleting a custom domain released its name while services still pointed
at it, leaving them on a namespace the account no longer held.
Deletion now refuses with 412 when a service in the same account uses the
domain or a subdomain, including disabled ones. Service writes revalidate
authorization inside their transaction and hold a shared lock on the
matching registrations, so a delete racing a create cannot strand either.
The dependency lookup is account-scoped: registrations are unique by name,
so another account can hold team.example.com under example.com and its
services are authorized by its own registration.
Maycon Santos