They are available to my own birds - I just don't rely on them having any reliable pharmaceutical benefit.
/edit to add/clarify - I grow these for my own culinary uses, I didn't plant specifically for the chickens benefit, and would not invest in the extra expense if I didn't use these things...
I can walk you thru the research. The theory is sound. The problem is the math. The concentration of the bio-effective chemicals in those plants is so low that to have any significant, measureable, long term health benefit your chickens would have to eat huge quantities - and they won't. And...
The gross numbers (Protein, fat, fiber, calcium, phos, listed aminos, etc) are fine.
The long list (below) is part of a premix, which suggests the vitamins are fine.
vitamin A supplement, vitamin D3 supplement, vitamin E supplement, riboflavin supplement, calcium pantothenate, choline...