Unfollowed a friend due to some strain on the relationship, and just needed to avoid them temporarily and didn't want to see anything from them. They shared some of my content, which placed them in my notifications.
Notifications can only be marked read, not deleted, so it was somewhat stressful. it's a very minor use-case, but there is no remedy but to get other interactions to force the notification out of sight.
Exposing more controls is always a balancing act. From a user experience perspective it can add complexity to the product, this can make it feel more confusing. The privacy setting, for example is very powerful but the UI is built so that most complexity is hidden until you need it. There is also engineering debt. Once a feature is added it is very hard to remove. A setting like hiding notifications on a per-user basis would have to stay in the codebase for all time, even if Facebook decided to stop showing the UI to new users.