This question comes up often.

Is our food becoming less nutritious?

Yes, but not entirely for the reasons you might suspect.

Soil quality has degraded over the past 80 or so years, so that certainly has been a large factor.

However, a less obvious factor has to do with the plant genetics that have given rise to higher yields in many crops.  The net result is grains with higher percentage of starches which dilutes the nutritional value of the crop.

Quantity over quality.