>What I am questioning is the why. Why did it stop.

Where do you even start? For a kid raised on phones, asking them to stop using their phone feels like asking them to cut off their hand. I imagine that most schools do have anti-phone policies, but there's a point where it's just too hard to enforce. Like other people have also mentioned, parents often pressure their kids to have phones in high school. As someone who went to high school after smartphones became popular, my mom would get upset when I want to school (or anywhere) without a phone on me. That means that teachers have to deal with a student body that's completely unwilling to follow the rules and parents that will back them up if you ever try. I certainly wouldn't bother enforcing a phone ban if I was a teacher.