Oh, I was responding to OP with that, not you! I suspect I might know where you are getting some of your info from.
I throw 24% around all the time - because I have ducks. and diets over 24% crude protein, particularly in very young ducks, are associated with higher incidence of "angel wing", which is not good. High Carb diets, the same. The pathology is complex and seems to have a number of inter-related factors.
For chickens, there's really just no benefit. I was looking for a specific study I've linked before, whose conclusions (briefly and broadly) were that increasing crude protein from 16% to 18-20% was associated with a 1-3% increase in rate of lay, a 1-3% increase in egg size, a similar decrease in mortality rates (assumedly due to greater vigor), and of course some increase in overall size. But considerably less gain, typically, than was offset by increases in feed costs, which then (much as now) grew about 10% to reach the higher protein level. The study looked at higher protein diets still, and found the gain from 20% to 24% was more like 1-2%, while the cost jumped again.*
My own personal feeding regimen is based (loosely) on feeding my dual purpose birds more like Broilers than layers for the first 8 weeks or so - it does no harm, it has some benefit, and I'm going to eat most of my males - so the extra expense of the higher protein feed is partially offset by the reduced time to table. But as a maintenance level, all the AAs (and by extension, protein) they don't need as their nutritional requirements drop would simply be excreted in the form of waste nitrogen - meaning higher urates and more ammonia.
* I should note, in fairness, that the conclusions of that study differ from the one I linked in this thread above, which found a 10% increase in body mass between the very low and the very high. The one I was looking for showed an increase in the 5-6% range, if I recall correctly. The point remains, though the details vary a bit.
