pf2e.fyi
From Pathfinder Lost Omens Shining Kingdoms, which isn't enabled.

Crowd Mastery

FEAT 6
Archetype Lion Blade
Prerequisites Lost in the Crowd

You use the crowd to your advantage, diverting your foes' attention to potential hidden dangers in the crowd. When determining if your foe is flanked, treat all squares occupied by a crowd as if they were occupied by an ally with a melee reach of 5 feet.

Pathfinder Lost Omens Shining KingdomsORC