What happens to a deer if they go above tree line?

Your last question I gave a joke answer. This one i'll be serious. Deer don't go above the treeline very often for two reasons. There's nothing up there for them to eat and there's no cover.

They probably do when they're hiding or scurrying away from predators. Usually when deer run away from me, they go straight for the trees.

I expect it's the food thing, but more importantly, it's probably a predominant trait to prefer to eat out in the open rather than in the forest because their ability to get away from predators relies on their ability to notice them coming.

If the predator has to cross an open field to reach the deer, the deer will notice them more readily and then be able to run, and if they notice before the predator gets too close, they can get away.

Because their food sources aren't up there mainly.