Except for very few specific exceptions (namely Ziggs ult), all abilities with horizontal travel time are blocked.
If there is a time where the ability can be seen as having already left the caster's hand/tentacle/weapon/etc, but it has not yet reached the point where it will affect the target, then it's probably being treated as a projectile, and will be blocked.
Every ability that is blocked by Wind Wall fits the definition of a projectile as it is used in League, even if thematically it makes no sense. In my opinion the two worst offenders are Vel'Koz's W and Viktor's Augmented E, specifically the line of explosions that follow the laser. Energy being channeled through the ground. It doesn't make much sense that they can be affected by the wind.
By the way, Vel'Koz's E "disrupts a nearby area", but it does so by sending a pulse of energy above-ground to that location. That's the projectile being blocked, and actually vaguely makes sense, as long as we're willing to accept that energy has enough form or mass to be diverted from its course. Which we must if we're to believe that Lux's Q and E can be blocked, or any other projectile that seems to be composed entirely of magic and light.