Yasuo's wind wall reads "Creates a moving wall that blocks enemy projectiles." I noticed the other day watching a game, that a Yasuo player threw up a wind wall but threw it on the wrong side of themselves (the enemy was the other direction). They then ran to the other side of the wall, making me wonder if it blocks both ways or just one way (and they were just panic running).
It seems to be curved very slightly on the ends, but that could just be a graphics choice.
The guide video only shows Ashe shooting from a single side.