Only you know the road you live on and the type of child you have.
It is not necessary for an 8y to be allwed to play out on their on out of sight on busy streets. He'll be fine.
My DD has just turned 9y and does play out and has done for the last couple of years (when she was 7y I still looked out regularly). However, we live on a very quiet cul de sac on a very quiet estate. There are several children the same age and younger. They play on scooters, gocarts and bikes, or run around playing. We have an L shaped cul de sac so and we live at one end. If they play the far end I can't see them. This has been fine.
Thi year, actually since the day after she urned 9y coincidently, she has been allowed further afield. If she is with likeminded children she is now allowed to go to the park on our estate. It iinvolves no roads and is just behind the houses. there are exits onto country roads and fieds but not right next to the playground bit. She has also been round to another friend's house to call for her - again with another friend with he. This involved crossing two very quiet estate roads.
However, DD is also a very sensible little girl who I can trust to follow the rules. She can tell the time and wears a watch so she knows what time to be back for. She has proved herself to be so. She always comes back to ask if she can play ins omeone's house or back garden, always asks if she can g to the park, always coes back at the time given - sofar, int he last month and half she hasn't let me down.
It really has done her a world of good. She has made lots of new friends - she doesn't go to the local school, so didn't have school friends here. She plays out far more than she ever did and is using her bike 100% more often than before! And we have got to know more parents too.
But - it depends on where you are and your child.