I think this is how it should be. Once the alert is sent, no one but the recipient should have access and control over it no matter origin exists or not. It is same as if you sent someone an email asking if he/she wants to join you in a hangout. Later you canceled your plan for the hangout and expect that if the email is not already seen, it should be removed from the recipient's inbox as if it never arrived.
If you get such alert and the user deleted the post, you click and enter the direct link to that post it should show you message "This post as been deleted", since when users delete posts it only removes it from public view. ^^