It's not going to make them drop dead, thus the clarifier "can"
But there's no magic "herbal properties" that make them a good thing to feed chickens.
Garlic doesn't prevent worms nor provide any super vitamins to improve their health.
Melding chickens and onions only in the kitchen.
Alliums(onions/garlic) can be toxic to chickens.
I'd never feed onions to my birds.
A good chicken ration and fresh plain water is about all chickens ever need.