Forum version 3.2.8
Member A belongs to Usergroup A that has access to Forum A. They subscribe to forum A and receive emails as expected. Six months down the road, they are no longer part of Usergroup A, and therefore lose access to Forum A.

They did not unsubscribe to Forum A. They still do receive email notifications on new topics created in Forum A, though, after no longer having permissions to view Forum A, the permissions being thoroughly verified.  They click on the links in the notification email and get the not authorized notice, but still do receive the email notification, complete with topic subject.

This is a problem. Whilst they can't actually access Forum A by clicking the notification link in the email, it still does send a notification and the topic subject.