I think it's a great idea. When a rider was thrown off his horse in those times they were injured and some times killed by the fall/throw.
Unfortunately, I don't they can program that in game-wise. The only thing that causes damage are the tips of weapons and arrows and bolts. There's usually an invisible box
at the tip of a weapon and when it comes into contact with someone or something or its bounding box the thing being hit takes damage.
The fall won't give it damage cause you're not being hit, unless when someone swings at you or thrust through you and the horse and kills horse and rider. I've seen that happened.
But, you're talking about falling damage. They could take falling damage as if falling from a very high wall and taking damage. That happens. But, again, they would have to program it to where when falling of a horse it triggers high falling damage.
That would be great, would be cool and it'll make it more realistic and less gamie/gamy(game-y) Cause cavalry just ride around like nothing and pick off people easily and take no
damage when someone finally brings them down. The rider just gets up(especially if it's an armored rider) and keeps killing without injury from the fall.
Good suggestion.