There's not many past work colleagues I would consider contacting. Particularly not if they have made no effort to contact me.
I've lost touch with some work colleagues that I would be happy to see again, but owing to issues keeping in touch, which is partially my fault for not remembering to backup my contacts when I wiped my phone, I haven't kept in touch.
If I bump into people and they are pleasant enough, I will reciprocate.
However there are a bunch that I'm happy to never see. They helped contribute to the most toxic work environment I have ever been in. They were also rather intrusive, insisting I add them to social media that I didn't have and they wouldn't accept I didn't have social media. Then also insisting I socialise with them after work, then behaving like a bunch of toddlers when I said "no". One occasion I was told that "we are going for lunch at [Expensive Restaurant]" I declined as it cost more than my weeks lunch budget. I was then basically interrogated as to where my money had gone since I had been "paid last week". It was like a overbearing parent demanding to know how a child had spent their pocket money.
I dunno, I guess I'm one of the few people these days that doesn't consider themselves as the property of the company. I also value my alone time rather highly.
I think it was Coco Chanel (not sure I've spelled that right) who said "Never posses something which you find neither beautiful or useful." I have a similar policy on ex work colleagues!