Can you activate a sacrifice outlet during the declare attackers step to remove a creature from combat?

Yes, you can activate a sacrifice outlet during the declare attackers step (or any point in combat where players have priority) to remove a creature from combat. When a creature is sacrificed, it is put into the graveyard as a state-based action and ceases to exist as a game object. A creature that no longer exists cannot be an attacking or blocking creature.

According to CR 506.4, after attackers are declared and any triggered abilities resolve, both players receive priority in succession. This means you can activate a sacrifice outlet at this point. CR 509.4 confirms that a permanent that is removed from the battlefield is also removed from combat.

This technique is commonly used to deny an opponent the ability to assign combat damage, prevent a creature from taking lethal damage, or to trigger 'when this creature dies' effects at a strategically chosen moment during combat.

Concrete Example: You attack with a Doomed Traveler. After attackers are declared, your opponent taps a creature to block. Before damage, you activate Viscera Seer's sacrifice ability, sacrificing Doomed Traveler. It leaves the battlefield, is removed from combat, no damage is dealt, and Doomed Traveler's death trigger creates a Spirit token.

Note that if the creature was already declared as an attacker, it will have dealt no damage since you acted before the combat damage step (CR 510.1). The sacrifice outlet must be usable at instant speed for this to work during combat.