It's well known that cattle kept inside ends up with much lower Vitamin D content in milk & meat.

Not sure about other nutrients.

There is research to support it. It makes sense as well. If your human diet affects your health/nutrition/body composition, animal diet should affect their health/nutrition/body composition.

https://core.ac.uk/download/pdf/14423473.pdf

Not the person you replied to, but I think "animal's diet affects their nutrition" is pretty obvious. For instance, the "omega 3" eggs you see at the supermarket are produced by feeding the chicken a high omega 3 diet. That itself isn't really interesting. The real question is whether going through all that trouble is worth it. In other words, is it better/more cost effective to buy the higher quality animal products at the supermarket (eg. omega 3 eggs) or buy the regular stuff and pop a multivitamin? For omega 3 eggs at least, the answer is the latter. I'd be interested in seeing how it works out for beef.

There are some measurable differences in certain vitamins, fat content,and amino acid profiles, but I do not believe the differences have a causal link to overall human health i.e. longer lifespan, improved health markers, etc. At least nothing significant in the literature.