He was an old friend who stood up for me in school when I was facing harassment. He had a tough persona, but he spoke softly to me and was gentle with me. That was in 2006. I messaged him on Facebook a month ago, nothing too detailed, just asking him if he was at [school] in 2006. To be specific, the exact message was “hey, were you ever at [school I went to] in 2006?”. I have a picture of my face as my profile picture, and my intention was just to re-connect with old friends or maybe to just thank him for what he did. That day, I was being harassed so badly that I was in tears. I’d been bullied a lot, and he was someone I looked up to. Today, his profile in my Chats list just says “Facebook user” and how he’s not available on messenger. My sister can’t find him on Facebook either.
Unless your schoolmate is here, I don’t think you will get a definitive answer. And what people do on social media is not necessarily representative to what they would do in real life. So don’t take this block personally.
A mixup of names has been suggested a a possible explanation. It could just as well simply be a mistaken touch on the app or a misplaced click on the site. That person may have their own ghosts of school life past to fend off and they’ve chosen to make a clean break and not keep in touch with people from that time for their own mental health. Or they are friends with the people who used to bully you and doesn’t want them to spot you on their friends list. Or, while they treated you nicely and stood up for you, it’s still possible they just don’t like you regardless. There are many possible explanations.