None of the above is true, or at least isn’t the answer for why today a “2x4” is missing an entire half an inch all the way around. The shrinkage due to drying is around 5% (the real answer is more complicated, and wood shrinks different amounts in different directions relative to the grain), which would only account for 1/10" of difference in the thickness of a 2x4. With some species of pine it’s as low as 2%.

No, the lumber industry has consistently shaved boards in order to fit more into rail cars for transport and make more money and spend less per plank on transportation costs. Various lumber consortiums determined via internal testing that the smaller board sizes are still “sufficient” for their intended purpose vis-a-vis structural integrity of stick framed residential buildings.