Yeah, this is not happening. I don't think a world conqueror + decree would do it, no one would do it and it'd just be the end of the world ruler. I suppose the mostly likely route might be a nuclear war or something. Chernobyl has apparently turned into a nature preserve since the nuclear meltdown 30 years ago (1). But even radiation probably couldn't keep people out if it was half the world we were talking about.

1: https://news.nationalgeographic.com/2016/04/060418-chernobyl...