Yasuo had been nerfed multiple times and he's still problematic none of them addressed real issue
##TL;DR: He can use windwall to nullify many different hard cc, and it lasts way too long for proper punishment
The reason champions such as 
can remain balanced despite having too much damage is because they are gated by hard cc. On the contrary, 
has windwall for self-peel and occassionally used for peeling allies as well. And it lasts 4 seconds. Compare it to defense abilities of other squishy champions:
's spell shield: lasts 1.5sec, blocks 1 ability, cd at 22/19/16/13/10 sec
's spell shield: lasts 1.5sec, blocks 1 abliity, cd at 20/18/16/14/12 sec
's reposite: lasts 0.75sec, rooted for 0.75sec as well, no limits on number of parried attacks, cd at 24/22/20/18/16 sec
's alpha strike: lasts less than 1sec, untargetable for that duration, cd at 18/17/16/15/14 sec
Any champion with a dash or blink: mostly has cd or is situational, and actually needs to think about the right timing. Some such as 
need to get into enemy's face for cd reset and risk getting exploded.
all of the champions listed above have some sort of choice to make. Some have to decide whether to use their defensive ability against hard cc or against damage. Some have to decide the precise timing despite being able to defend more than 1 ability. And some have to expose themselves to reset their possibly defensive ability (dashes and blinks).
And here's the last one
's windwall: lasts 4 seconds, rooted for 0.25sec duration, blocks as many projectiles during its presense, cd at 26/24/22/20/18 sec
Yasuo's windwall is simply way too forgiving with poor usage; he does not really fit into any of the above that i listed regarding defensive ability usage. He does not have to think about which specific enemy ability he want to block. That's fine as long as it is gated by timing. So how long does it last? FOUR FREAKING SECONDS. LONGER THAN A MAXED MORGANA SNARE. He can use it too early, and it is too hard to punish him as well for it; its width is big enough if enemy is pretty far away, trying to walk around the windwall only results in giving him too much time to disengage. If enemy is close enough to walk thru the windwall, yasuo can practically hug the windwall and do free damage.
##The real nerf that he needs is windwall's cooldown.
can do true damage for the entire duration of 1vs1 or 1vs2 and still be balanced because there's plenty room for punishing him with hard cc.
could only do highly-situational, partial penetration that doesnt even come close to true damage and still be considered problematic because there is not enough room to punish him with hard cc.
If windwall gets [season 6 malzahar passive]'ed, then it would give more room to punish yasuo for misusing windwall. Even if all his abilities and basic attacks do true damage im exaggerating, u get the idea, it would not be as problematic this way.
