> Such a system would presumably account for possible diversions and plot your flight accordingly.

I mean, that's fine if you want to avoid Russia while flying from California to Quebec, but you don't really need an app for that one. Whereas if you're within the plane's fuel supply of where you don't want to be, how are you supposed to know ahead of time what kind of nonsense is going to happen while you're in the air?

The plane could have a navigation failure over the ocean and end up arbitrarily far off course. Some first class VIP could have a medical problem which is going to force the plane to divert anyway and then the nearest city with the right kind of hospital is in the place you don't want to be. And what if you end up St. Petersberg not because you had a layover in Finland but because Helsinki was your intended destination?

You can't account for any possible contingency obviously, but you can still account for most. Start with just determining possible diversions on the route as it is supposed to be. That route can in turn be replaced with a heatmap of historical routes the planes on it have actually taken. And so on. The point is to not be on a plane that can be diverted to a country where you really can't end up at any likely point of its regular route.