Nothing new here. Our customers opened the window/exit quite often in our DC3s, doing the Arabian Gulf milk runs back in the days. On the ground and occasionally in flight. It wasn't difficult; the exit was hinged at the bottom and all that was need was to unlock the handle and push.
The practice pre-dated the presence of cabin crew, but continued after they became mandatory. Sometimes the steward would notice a window was open, sometimes not.
I have to admit, though, that no-one actually stepped out on to the wing, on the ground or airborne.