I almost never see it now, but twenty to thirty years ago when there where far more cars on UK roads with no air conditioning, you would very occasionally see someone leave the window slightly open to try and keep cabin temps a little more sane in summer. In the 90s/early 2000s there where a lot of cheap cars (Ford Fiestas, Corsas etc) sold in volume on UK roads with no AC, and on hot days those things would turn to ovens if parked in the sun with the windows up. I used to have to do this occasionally myself with a crap Ford Ka I owned with no AC. It was a pretty normal thing to do back then!
I'm not even sure if it's possible to even buy a new mass-produced car with no AC anymore in the UK, or pretty much anywhere else for that matter.
Unlikely to be normal, but I have been doing it at a couple of places I park at this year, since I'm there for a while, have to park in the sun and the places are semi-enclosed (ie, it is fenced so random people not walking around but not secure compound level). Also, my car is 17yrs old, I got it for free and don't keep valuable items inside. I prefer leaving the window open to coming back to 38degC inside. No aircon.
The US is huge. In hot dry rural areas it's also perfectly normal to park with the windows down to avoid the interior getting even hotter. (And theft is not really a concern.)