What temperature is ok to leave the deck in the car ?
The temperatures in locked cars in full sun can easily kill pets, kids and even adults, so I imagine that is not so good for the wood in skateboard decks either.
Some people say "Just don't" but I guess it depends if you are parked outside in full sun all day every day, or if you have things covered, insulated or just sitting in the back.
A friend who used to always complain about his boards delaminating had a sporty looking coupe with a big back hatch and that is where the board lived, never took it out except to skate and didn't cover it with anything. He never listened to any of us but when he wrecked that car, sure enough his boards never warped when in a normal sedan boot.
I recommend using a board box to keep any decks or complete skateboards in if you have them in a vehicle, as it insulates them enough to stop any of the usual issues with heat and my boards are good. That and covering anything if it is just in the back seat or visible, either a blanket or shade cloth material, more than anything prevents unwanted attention in the case of someone coming round looking in car windows and breaking in to steal anything.