I've recently quit my PhD after having completed 2 years of the program. The main reason for quitting was the ridiculous behavior of my supervisor- he had zero experience in my area, constantly put me down & discouraged me, and the University lacked any facilities I needed for my research.
I want to ask if it's normal behavior for a supervisor to be so "obsessed" with one of his ex students that he goes through lengths of unfollowing me and even BLOCKING me on social media, months after I left? To me this seems really childish and mentally unstable behavior, which doesn't even happen if you've ended a romantic relationship! It makes me feel a bit creeped out since it obviously means he is thinking about me even months after!
Has anyone had any similar experiences/ thoughts on this behavior? Is it normal?! He has of course not erased my publications!

Unless he's actually harassing you or doing anything to cause problems for you, I would ignore it. People do sometimes have a 'purge' of their social media to unfriend/unfollow anyone they're no longer involved with, so it might not be personal to you (I admit blocking does seem a bit odd, unless it was a mistake and he just meant to unfollow).

I guess supervisors are human too, and not all humans are rational, pleasant people.

I just want to point out that that behaviour is very common at the end of romantic relationships, has led to multiple homicides via the way of domestic violence and other tragedies. I'm just pointing that out because while this is a postgrad forum, everyone on here is still a person, some who may unfortunately be in abusive relationships, or have had history of being in one, or whose parents may have been. So to say that it 'doesn't happen in romantic relationships' is incorrect. It's just that it's not talked about or recognised as much as it should be.

In saying that, the behaviour of your supervisor is peculiar, however, as TreeofLife said, that supervisor is a human as well, and will react in human ways. My one concern is whether or not the supervisor has 'pull' and will use that to deter any future plans of yours in an academic career. That can and does happen unfortunately.