Why would someone who is engaged friend their ex on social media?

Many reasons. Some because during their relationship, they were friends more than lovers. Some curious as to see how their lives are turning out. Some miss them. Reasons why can go on forever. Only way to know is to communicate and ask. Asking is better than ruining your days with unknowing and checking both parties media pages to see who likes who's posts and pics.

It depends. I dated a lady starting in 1980 for a few years. It turned out that marriage was not in the works; but, we were great friends. 41 years later she is my best lifelong friend. My wife knows all about her and is happy that I have had such a long term friendship.

Just because they are ex's from a romantic perspective, does not mean that they cannot be good friends otherwise.

Now there are people I have dated that it all ended when we decided we were not compatible.

I'm not interested in being friends with my exes.

I was friends with my exes after a few years it's normal in my opinion

Bad sign from my own experience.

Why not?

This is usually a sign that they are having second thoughts about their engagement. It's possible that they are trying to see if they have another option before fully committing to the person they're engaged to.

Theyre trying to see if theyre down to fuck