As someone who has earned a living from animal nutrition for around 40 years, here I are my brief thoughts:
All animals require protein. Protein is just nitrogen in varying guises and can come from a variety of sources such as green material (grass, etc), plant materials (cereals, soya, peas, beans, etc), man made materials such as Urea which is 297% protein and £400 tonne compared to 25% protein in peas or beans which are worth around £100 tonne.
So you can already begin to see that man made proteins like Urea are miles cheaper per % Protein that more natural sources. It's not quite as simple as that but you get my drift.
Poultry manure is typically 20% Nitrogen so that's about 125% Crude Protein and , as a waste product, is free or very low cost so you can see the financial appeal.
In commercial agriculture it was promoted heavily for use in beef rations around 25 years ago. As another poster mentioned, it needs to be treated (basically a form of composting) before it can be fed. My experience is that farmers who trialled it never came back for seconds. One problem was a high level of food waste in ad-lib systems so most of the cost savings ended up being shovelled out of the feed passage and onto the compost heap. The other problem was market resistance. The idea of eating beef fed on poultry s...t was a hard sell.
As a result, the fad came and went in less than 3 years.
Is it still legal ? I honestly don't know.
Does it fit into smallholder ideals and ethics ? I can't see how it can
Interesting talking point nonetheless