If yasuo remorseful attacks a battlefield with akali silent, can he target her with his ability?

Yes — Yasuo, Remorseful can target Akali, Silent here.

Akali's passive only protects her while she isn't in combat, and attack triggers are placed on the chain after Attacker/Defender designations are assigned (Rule 383.4.e.2; 464.2.c.3). So by the time you finalize Yasuo's trigger and pick "an enemy unit here," Akali already holds the Defender designation and is in combat — her restriction is off and she's a legal choice.

This is the opposite of the "When I move" trigger case (e.g. Akali, Deadly Weapon), which fires before combat ever opens and therefore can't choose her.

Two follow-ups worth remembering: the damage is equal to Yasuo's Might at resolution (6), and since targets are picked at finalization, an opponent moving Akali to base in reaction would make the trigger mistarget and deal nothing (Rule 359.3.f.2).