Earlier quoted context omitted.
My bank sends (non-transactional) email to me with an unsubscribe link, then magically I get sent more even after going through the whole thing (to the screen where they confirm everything has been unsubscribed). It's hard to confirm externally that things worked.
Google probably tracks all clicks on links in mail. It is easy to see that unsubscribe was clicked, and then more mail that you mark as spam came.
It's nudging me towards switching banks.