In a paper titled "Falling Into Straw," physics students at the University of Leicester debunked any hope you had in the myth of leaping off a building and landing safely in a haystack in Assassin's Creed.
After what I'm sure amounted to vigorous research into haystack leaps in the game, the students concluded that, put simply, you shouldn't do that IRL. While the piles of loose straw would provide some cushioning from falls, you couldn't pull off the kind of jump an assassin does because the haystacks are too small. The haystacks in game are only about 1.5m tall, and according to the paper, you could fall no further than 12-13m and expect that stack to save you. If you're willing to sustain "severe injuries," you could stretch that height to about 50m, but you certainly won't be shanking any enemies after that leap of faith.