My Yasuo Remorseful got stunned, does his ability work? Deal damage when attack?

Yes, your Yasuo, Remorseful's ability will still work while he is stunned.

According to RiftJudge FAQ #7935 and FAQ #8745, stun only prevents combat damage—it does not prevent ability triggers or the damage dealt by those abilities.

Here is the breakdown of why this works:

1. Stun Mechanics: Rule 423.1.b states, "A Stunned Unit does not contribute its might to damage in the combat damage step." This specifically limits combat damage, not ability damage.
2. Ability Triggers: Stun does not prevent a unit from triggering its "When I attack" ability.
3. Resolution: When Yasuo's ability resolves, it checks his current Might and deals that amount of damage to the target. Because the stun does not affect ability damage, the full amount of damage is dealt.

Important Nuance: As noted in FAQ #514, this only applies if Yasuo is the attacker. If Yasuo is stunned while he is the defender, his "When I attack" ability will not trigger in the first place, because he is not attacking.