The red zone has pretty much been cleared in urban areas. I actually live exactly in it, west of Lille, and all the area is either built up or farmed.

I believe it's mostly in the more sparsely uninhabited Aisne and Ardennes that there are still no-go zones. In the Nord they are less common (though they do exist, like around Vimy).

I think WWI leftover ammunition in the north is actually less a problem today than all the unexploded US and British bombs from WWII (which are usually around city centers).

The red zones can be inhabited but most of the ground is either polluted either completely destroyed. And as the boards said some zones still have armed explosives in them. I went there once. It's pretty beautiful when you forget it has been caused by a useless and slaughtering war.