Part of the decrease is likely to be due to slower speeds, fewer children playing out unsupervised/just with a sibling. When I was 4 in the 70s I was able to wander about 500m away completely unsupervised.
I think that as with most aspects of parenting it is about weighing the risks and gradually passing over responsibility to your child. You don't suddenly say at 18 that they are totally independent having been totally dependent on you the day before. There will be a gradual handover, leaving them in the room on their own, clearing their plates, walking ahead of you, going to a party without you, having a phone, going to the park with a friend, walking to school alone, staying at home while you pop to the shop, staying home alone while you go away. It is an ongoing dance of judging when they are ready for the next step.
Sometimes they show that you can trust them more so you give them more independence/ responsibility, sometimes they do something which demonstrates that they can not be trusted with the responsibility so you drop back to the previous level of responsibility. Some children never need to wear reins, others will not need them when they are 2, others are older.
The art of parenting is giving each child enough freedom and at the right time to have the possiblitiy to fail a little but not so much freedom that they are totally unsupported or in real danger.