Is it safe anywhere for kids to just go outside and play without adult supervision?

Kids play. Yes and no. It depends on many factors and circumstances and the ages of the kids.
Age, Buddy system, Age and location of play are definite considerations when assessing the safety of kids playing outside by themselves. Having a buddy system is highly recommended. Even if you are not directly supervising younger children, you still need to keep an eye and ear out. Review safety precautions regularly, without being alarmist.