"I also wonder about the long term effects of a leash (some people still use them"

By leash, do you mean a chest harness and walking reins or one of those curly straps that go from the hand of the adult to the wrist of the child?

I used one of those extending dog leads on Limpit - not only did it not impair her motor skills, it saved my back as the reins alone were too short for her to toddle far enough away that I didn't tread on her. I refused to have one of those curly wrist straps because I felt they were too flimsy to be of any use or liable to dislocate a shoulder or wrist. If anything impairs motor skills it's keeping children in pushchairs and buggies far longer than necessary because it's easier to hang shopping on or quicker than going at the child's pace.

LTS