When Yasuo Remorseful moves into a battlefield, and a hidden card for example, fight or flight reacts to it to move it back to its base, does the "When I attack..." trigger trigger?

Yes, Yasuo (Remorseful)'s "When I attack" ability will trigger, but it will fail to resolve successfully.

According to RiftJudge FAQ #8014 and #4699, here is the sequence of events:

1. Move: Yasuo moves to the battlefield, which triggers his "When I attack" ability.
2. Trigger: Yasuo's attack trigger is placed on the chain.
3. Reaction: The opponent uses Fight or Flight (played from hidden) to move Yasuo back to his base.
4. Resolution: When Yasuo's attack trigger attempts to resolve, it checks for the target "here" (at the battlefield). Because Yasuo is no longer at the battlefield, the ability has no valid source location to evaluate, the target becomes illegal, and the ability whiffs (deals no damage).

Key Rule: As noted in RiftJudge FAQ #1545 and #948, Riftbound abilities recheck their conditions upon resolution. Because Yasuo's ability specifies "here," he must be present at the battlefield when the ability resolves for the damage to be dealt. Since he was moved to his base by Fight or Flight before the trigger resolved, the condition fails.