So falling states that the max dice rolled for damage is 20d6. So is anything beyond that just considered an instant kill? I know it’s unlikely that a character is higher than 200 ft in the air but in that event I want to be prepared.

anything above 200ft is straight 20d6 damage, consider it terminal volocity, it doesn't matter how much higher than that you are you are still going to hit the ground with the same amount of force.

Oh yeah that makes sense 

Terminal velocity on Earth for a human that tries to have maximum air resistance is approximately 125 mph and is reached after around 1000 ft. free fall... D&D probably simplified that a lot.

D&D has capped falling at 20d6 since first edition. Of course, in first edition it was very rare to have more than 70 hp.

There are multiple stories of people falling huge distances and surviving.  Usually landing in snow or being slowed down by trees or something.   But it is totally possible for someone to fall 2000 ft and survive.  Unlikely, but possible.

20d6 averages about 70 hp.   for a D&D world, a hero >70 hp should survive the fall