The corollary to mutual assured destruction is that Russia can use a nuke in a regional war not involving a nuclear power because they know no power will retaliate for fear of triggering full out nuclear war.

What keeps them from using in a regional war is social pressure from other peer countries. They won't use a nuke in Ukraine because other countries would see it as crossing the Rubicon and isolate them (also Ukrainians are basically Russians).

But if the West almost totally isolates Russia so they can only trade with China or 3rd world shitholes, then this pressure is gone and they can freely use nukes.