Does Yasuo Remorseful moving to a battlefield trigger Fight or Flight to his ability (which I believe is possible), and does this stop Yasuo's ability going off?

Yes, you can stop Yasuo, Remorseful's ability from dealing damage, but only if you play Fight or Flight as a [Reaction] in response to his ability on the chain.

If you play Fight or Flight as an [Action] from your hand during an open state, it is too late; Yasuo's ability will have already resolved.

Why this works

According to RiftJudge FAQ #3371 and FAQ #8014:

• Yasuo's "When I attack" ability triggers and is placed on the Initial Chain when he moves to a battlefield.
• Because the ability is on the chain, you can respond to it with a [Reaction].
• Fight or Flight can be played as a [Reaction] from a hidden state while the chain is active.
• When Fight or Flight resolves, it moves Yasuo to his base.
• When Yasuo's ability subsequently resolves, it checks for the target "here" (at the battlefield). Since Yasuo is no longer at the battlefield, the ability fails to resolve (it "mistargets").

The Sequence

1. Yasuo moves to the battlefield: This triggers his "When I attack" ability. The ability is placed on the Initial Chain.
2. You play Fight or Flight as a [Reaction]: You must play it from its hidden state in response to Yasuo's ability.
3. Chain resolves (LIFO - Last In, First Out):
   • Fight or Flight resolves first, moving Yasuo to his base.
   • Yasuo's "When I attack" ability attempts to resolve. It checks for the target "here." Because Yasuo is no longer at the battlefield, the ability has no valid target and fails to deal damage.

Important Distinction

As noted in FAQ #3371, if you attempt to play Fight or Flight as an [Action] from your hand during an open state (after the Initial Chain has already resolved), Yasuo's ability will have already triggered and resolved, dealing its damage before you have the opportunity to move him. You must use the [Reaction] timing to interrupt the ability before it resolves.

Note: I could not fully verify the claim that "[Hidden]" is a keyword on the card, only that the card can be played as a reaction from a hidden state.