I was born early 80s and my parents were v over-protective - e.g. I wasn't allowed go on sleepovers, strict curfew, etc etc.
They would have dropped me into the 'creche' at the supermarket, which was basically a small area with a sandpit and toys supervised by some teenagers. That was fairly standard, though people clearly took the piss: it was obviously meant to be while you did your 'big shop' so for an hour or so but you'd hear them calling for parents on the tannoy and people clearly left kids their for hours. That was probably from age 4.
They would have left me in the toy section of a department store while they went to another part of the shop from maybe 7or 8, though not for long. Again, I think that was normal enough: but not leaving the shop, and not for long, I'm thinking 10-15 minutes because I didn't want to leave and they wanted to get on.
And they would have left me in the library for probably up to 20 minutes:they'd always take me on a Saturday and also pick up my granny's pension for her at the post office about a ten minute walk away, sometimes depending on the weather/timing,they'd leave me in the children's section and go get the pension. I may have been slightly older.
So... I think there were places considered 'safe' that you would leave your child for a short while, at an age where now you wouldn't let them out of your sight And it was probably more ok for kids to wander to have a look at the magazines/books/whatever section was more interesting in the supermarket. But I think the 'norm' (obviously dependent on location)was probably to do this for much shorter times and at older ages.
Equally, when I was 7/8/9 and my dad was picking me up from primary school he was often quite late, and once he really did forget me - he thought my mum was picking me up. It usually depended on if he got caught up talking to a client. The time he 'forgot' me I was standing outside the school on my own for 40 minutes. That wouldn't be acceptable now, and I was obviously the only one still there:I think that time a teacher came out. And it was normal enough for kids at 9 to start going home solo, at 9/10 I took the bus, my parents were reluctant but my main objection was dad kept being late. The bus was to my granny's house so I wasn't unsupervised and it was a v straightforward short trip but I don't think it would be done at that age now.