I don’t believe in such “rules”. My best friend has made it clear that if I ever dated any of his ex girlfriends (even the one he had like 20 years ago) he’d never talk to me again. This is because he considers his ex gfs part of his own life, and can’t see them as part of a friend’s life. Which makes pretty good sense to me.
I had a big fight with him when he dated someone else’s ex. She was not my ex, but another friend’s, and I felt weird, because I was used to seeing her with my other friend, not him. A lot of it also had to do with the fact that she was my flatmate and I loved her (as a friend) and he didn’t respect her very much.
Also, his brother dated his ex for a while, which as you can imagine made my best friend furious. They didn’t speak for 2 years after that. They’re ok now though (after the brother broke up with her).
But I think the biggest problem is that if someone is close enough to you, they already know a lot about your ex. And you know enough about her too. And there’s some reason why she’s your ex, which quite often has to do with things your friend should know and avoid. So I’d never date any of my friends’ exes to start with anyway. And I’d warn my friends not to date mine, but only out of love for my friends. Unless the reason for breakup was something different and the ex was actually a good girl, in which case I’d be fine with it (I’ve actually passed on an ex to one of my friends).