If Yasuo moves to an enemy battlefield and there is a hidden Hidden Blade card, can the opponent react with the hidden blade card before Yasuo attacks and deals damage to a unit at the battlefield?

Yes, the opponent can react with the Hidden Blade to kill Yasuo before his attack ability resolves.

According to the Showdown Timing Flowchart and the provided RiftJudge FAQs (specifically FAQ #1545 and #948), here is the sequence of events:

1. Movement: Yasuo moves to the battlefield. This triggers a Showdown because units from different players are now present.
2. Trigger: Yasuo's "When I attack" ability triggers and is placed on the chain.
3. Reaction: Before that ability resolves, the state is CLOSED. The opponent can reveal and play their Hidden Blade as a [Reaction] to the attack trigger.
4. Resolution:
   • The Hidden Blade resolves first (LIFO - Last In, First Out), killing Yasuo.
   • Yasuo's attack trigger then attempts to resolve. Because Yasuo is no longer on the battlefield, the condition "here" cannot be evaluated. The ability fails to resolve and deals no damage.

As noted in the official rulings, Riftbound does not use "last known information." If the source of an ability that references "here" is removed from the battlefield before the ability resolves, the ability will fizzle.