I've been thinking about this and I think dividing games in linear or non-linear is too simplistic.
Skyrim for example has a giant open world where you can go wherever you want almost from the very beginning (there are some areas you probably shouldn't go to at low level, but you can if you want to). However, almost all of the quests and dungeons are very linear. Once you enter a dungeon, it's a straight path to the end while killing all the enemies inside. Similarly, when you start a quest, it's a matter of following the quest marker until the quest is done. There are a few exceptions, but most of it follows this pattern.
This is worse in games like GTA, where you get a big sandbox to play in, but as soon as you start a mission you have to either finish it or abandon it and the missions themselves are very restricted.
The first two Witcher games on the other hand had a pretty linear world, but allowed you to make meaningful decisions in the story that could actually close off entire parts of the game.
I suppose the most non-linear games are the ones that just give you a sandbox to play in, like Minecraft. Then again, you could argue that Minecraft isn't even a game, as it has no proper goal to work towards.