Nutrient degradation does happen with oxidation, as pointed out by
@MaggieJ, particularly with A and E. Other nutrients less so. Take the B vitamins, at room temperature they are fairly stable. Under high heat they degrade. Take, for instance, apples. When fresh they have a fair amount of B vitamins. Turning them to apple sauce loses about 50% of the B vitamins. Turning them to apple butter loses another 50% (~75% altogether). We accept those loses for the sake of preservation. Even so, those loses had to have hours of 212F.
Since only dairy products have governmental requirements of "best by" dates (and that only because the goverment believes us to be to stupid to know what soured milk smells like) we can be fairly sure that most of those dates are for marketing purposes. Empirically, even the 18 month deadline for home canning is not hard and fast. Yes nutrient losses occur, but I do not believe them to be that significant for most.