An 'unmissable' notification from Google wouldn't be anymore unmissable than the request email sent by the bank. Which is why the bank wanted Google to disclose the identity of the user. If Google deleted a users emails and disclosed their identity to a third party then Google would be in breach of their own privacy policy. They would be exposing themselves to litigation, hence the requirement for a court order.
Except that it could have ended up, possibly along with the original email, in a never read spam folder. Google has more direct means of reaching an account holder.
