My "Followed posts" list is littered with deleted questions, which I cannot remove off the list, as they've lost their "unfollow" option. How do I prune off these entries?
You can't via the Q&A page itself: see How do I unfollow a deleted question? on Meta Stack Exchange.
As @MassimoOrtolano indicated in the comments, it is possible now via the list of followed posts in your profile. However, you only see deleted posts there if you have 10k+ reputation, so this workaround doesn't work for everybody.