If Yasuo, Remorseful is no longer in play when its attack trigger resolves, what happens?

Ruling: Nothing happens. The ability returns a null value and no damage is dealt because Yasuo must be at the battlefield ("here") when his ability resolves.

Nuances:

• If Yasuo is moved away from the battlefield (e.g., returned to base via Hidden Blade even if saved by Zhonya's), the trigger still does not resolve and deal damage because he is no longer "here" at the battlefield.