Personal anekdata: I've seen much more unsupervised children in low-income neighborhoods than in upper-middle class ones.

just say anecdote

it's not an anecdote. an anecdote is a single story. this is his/her collection of anecdotal evidence/data. I don't see why they've spelt it with a k though

It's just a spelling mistake, I've meant https://en.wiktionary.org/wiki/anecdata

perhaps in the US, but wouldn't this equally have been the case historically?

the overall effect is almost certainly the result of the proprietors of playgrounds shielding themselves from lawsuits. I'd be willing to bet that around the time it started to change there was a big lawsuit, or a string of them that put the fear of god/bankruptcy into people running these things for the joy of others