Open world gaming is totally this normalised thing now that applies to every genre you can imagine: racing games, survival games, RPGs, shooters. But, as a concept, it's a relatively new thing. This YouTube video by Ahoy is a really comprehensive, definitive look at the Open World game. It's well produced and absolutely worth a watch.
It might be a little console focused for some of your tastes, but I found this made for some compelling viewing. Particularly the part about Hunter, an open-ended military sim, released in 1991 that I hadn't heard of.
Give it a watch, you won't regret it.