I have used DE since the mid seventies, mostly as a filter media for my aquariums. I am not a Vet nor a doctor, but I have seen DE listed on the ingredients label of feed, and as best as I understand it, it is used as a preventive wormer, or such. The amount is small.
DE is a somewhat hot topic, some feel it a cure all, some think it quackery at its highest level. Could be something like Chevron gasoline's newest additive, no one knows for sure what it does, but everyone wants it. The buzz out there may have created a 'need' that the feed companies wish to fill. I don't know, nor do I know anyone who does know.
Humans should not breath the dust, and I surmise that neither should chickens. Folks use it in their litter on the floor, in poop boards, dusting boxes and anywhere else someone thinks it might kill something. It's use is to kill things we mostly can not see … and there in is the problem. Faith does weird things to some people, and faith in DE is no different.
I have it, I use it. I don't go overboard with it in or on anything. It seems to work on stubborn ants. So in the feed, I suppose it is there to kill something. What that something is, perhaps we haven't been told yet. And as it is too small to see, well we are back to faith once again.
I could and perhaps would feed it as a complete lay ration. The big feed companies have most likely spent a fair amount of money and time to 'certify' that it is safe to use. But then Firestone recalled a ton of tires as well.
I don't know if I have helped or hindered with my thoughts, but that is how I view the DE topic as of now.
Best,
RJ