exactly. I am trying to keep in mind the difference between modern industrial layers, and backyard chickens. This thread is about ex-industrial layers, and started with some of them, but relatively few posts that I read on BYC are about industrial layers. By accident, ManueB had some that were rescued before facing a year in the unit, instead of once deemed 'spent', so they had the genetics and the pre-placement industrial feed for chicks and growers, and then they ate whatever she gave them from adoption. They could offer a rather rare insight into what difference it might make.
We made tons of nutrition mistakes in the beginning because we knew nothing ! We began by giving them the layer feed produced and milled by the place where we got them, but they were only three months old, so probably too young for a feed targeted for laying hens. Then, we mixed that with what my partner had seen his grand aunt do, which was give them all the potatoes too small for human consumption ...daily

. Also, we didn't realised milled layer feed had a pretty short date before turning stale, so we bought a year supply. In spite of that I can't really say for certain whether any of their health issues came from the food. I had six, and five are now dead : four died of various reproductive issues. They always had access to a small space to forage and they did a lot of exercise because our land is very steep.
It's been a bit less than a year since I switched to a feeding system where all the chickens can choose from layer feed, starter feed, a mash made with a mix of both, fermented grains, or dry grains. Three of my ex-batt's died while they were under this type of feeding. Once they were really sick, anyway they ate a lot less of any of those food, and more things like grass, scrambled eggs, fruits and anything they would accept to eat, really.
Honestly, I can't say what has been the impact of food on their health, compared to their living conditions, and their genetics, and I would also add the eventual stress from their social relations when I got more chickens, and a rooster, then another ... It would require more experience with keeping chickens than I have, and a more scientific approach.
What I can say is that they did a bit better health wise and in longevity than the same generation of hens from the same place who got out at 18 months instead of 3 and lived in a much smaller space at my in-laws.
My last ex-batt, Nougat, is now four and 8 months old. She is still one of my most prolific layer, laying five eggs a week. She actually lays more and with better egg shells than she did last year, so that can possibly be due to the change in feeding regimen- or to the fact that there is less stress in the group now that the dominant rooster is a nicer guy

.
I have no issue with commercial food in itself. I don't think it's the best option, but I think it can work if a chicken has access to foraging and other food and has good living conditions. I do have issue with saying it needs to be 100% of their diets and that it will make a chicken healthy, especially saying it in a very assertive manner.
Most people here don't give chickens "complete" food like layer feed or starter feed, and when they do, it's seen as a complement to grains, scraps and foraging, not the other way round. So I was very surprised when I first began reading posts on BYC, and it did impress me sufficiently that we gave less extras for a while.
I think sending upset beginners with ill chickens reliable links to either good BYC articles or other sources is the most helpful way to respond. I saw recently that a member who could be said very pro-commercial food sent this, and I think it's a very good start to evaluate if nutrition may be causing an issue :
https://www.msdvetmanual.com/poultr...ement-poultry/vitamin-deficiencies-in-poultry