I think it depends on the area, where they're going, what they're like, who else you know in the vicinity, etc.
Walking to a corner shop, or a friend or relative's house that they've been to with you a billion times before, in a safe area and where there are no roads to cross, 7 is probably fine for a sensible kid, maybe younger if it's just a few doors down. You could get him a watch phone with a tracker or something, but it won't really make him safer when he's young and just going to places he knows well. It will give you something to check but if you're prone to anxiety (a bit unclear from your posts) it may well make you more anxious over all, increasing your sense that it's dangerous to let him out of the house without you and you need to check.
To keep him safe you need to help him develop the skills he needs to navigate the world alone. And that's normally done by taking him places and explaining what's going on. Letting him watch interactions. Talking about how you decide what to do in different situations and when things go wrong. Letting him do stuff with you near by at first, paying for something at the shop, or returning a library book, say. Then while you wait outside. Then from further away, or while you go into a different shop. Going in a group of friends rather than on his own, especially the first few time he goes somewhere busy is also a good tactic.
I think it's a bit different today than it was 40 years ago when most kids would go to the park from a young age. There's less crime, but roads are busier and people often don't know as many people in their neighbourhoods, and there are fewer kids around. But it's not more dangerous, just a bit harder and probably less fun.
Going off without letting you know is a different issue, but it doesn't sound like you're asking about that, just planning for how to let him spread his wings a bit safely.