The cutoff is to stop the electricity from leaving the battery, and electrifying the rest of the car. Nothing more fun that getting electrocuted just touching a car. F1 cars are also Hybrids, and if you watch F1 drivers exit a the car after a crash, they do a little hop off the car. Making sure not to touch the car and the ground at the same time. They also have little indicators that show if there is current running into the car's body. I will venture that an F1 hybrid system is at least as advanced, if not more-so, than a common consumer hybrid system. At the least, I wish Commercial EVs had that same electrification indicator.

Cutting that wire is largely, to de-power airbags and the like. Airbags DO go off unexpectedly, and can inure a responder quite effectively. Cutting the power is a normal step in any big motor vehicle incident, EV or ICE.

Tesla and others do publish all this information with nice diagrams and handy mobile apps, and we study them. However, this doesnt fix the problem of the Huge Energy load in the battery itself. Even if that energy cant get out of the battery through wires, it can still burn like crazy inside (and outside) the battery case(s).