"We took all the armour off of Kayle's base and left her stomach exposed because it made her look tankier than she's supposed to be."
All they really did was slim down the profile, make her tassets less pronounced, and removed the belt buckle connecting them. She's still got roughly as much armor post-VGU as she did pre-VGU.
Proceed to give nearly every single skin more armour and properly defended vital organs.
I mean, Veridian Kayle has much the same armor set-up (cloth armor at the waist, plates most everywhere else), Iron Inquisitor only has armor on her shoulders, forearms, waist, and knees, Pentakill I won't count due to thematic reasons (metal artists don't really need functional armor), Silver Kayle has even less armor (shoulders, greaves, helmet - that's pretty much it) and Unmasked has less than that.
In total, I see 4 skins where the armor is plentiful (Battleborn, Aether Wing, Riot, and Judgement), against 5 skins with the same or less armor to the base skin. (Veridian, Iron Inquisitor, Pentakill, Silver, Unmasked).
So why does her base look the way it does?
For the same reason that Leona's armor consists of a skintight suit with armor plates and bars around it, Pantheon wears little more than a battleskirt, helmet, and a breastplate, and Taric exposes his gorgeous chest without a care.
Targon has never particularly given a damn how functional their Aspect's hosts' armor is. Being frank, Kayle's armor is a step up from the majority of the roster, only beaten by Diana.
An exposed stomach doesn't matter when you're literally an enchanted celestial battle-angel.
