There is a reason that most schools don’t have a blanket rule on this.
My daughters school is urban, centrally located in a fairly large town. Catchment area is probably around a mile all round; lots of busy roads but also lots of safe crossing points.
They don’t have a set age where kids can walk to/from school, because obviously individual children’s walks can be very different. My daughters is around half a mile, through “nice” areas, with no roads. Her friends is closer; but with several roads to cross.
Also, individual maturity and abilities of children can differ. For example, a neurotypical child may navigate their commute more successfully than a non neurotypical child, due to perception of danger. A “rule follower” may be safer than a child who likes to push boundaries. The level of practice that a child has plays a part too - my daughter is too young to walk herself, but we are already “training” her (eg we ask what she would do if the pavement was blocked? What if someone offers you a lift?)
Basically, you could have 30 children of the same age in a class, and for some of them, walking home alone would be a safeguarding concern, but for others it would not be.
I have reported a safeguarding concern about this exact situation, and it was taken seriously. The child was 7; and was running across a busy road without looking several times (then crossing back over to the same side - there was actually no need for them to ever cross a road) - several cars had to stop for them.
The report wasn’t because they were only 7 - the report was because they were unsafe in their behaviours. Had they not been playing on the road; I wouldn’t have called (although I certainly wouldn’t have let my child do the same walk at that age)