That is what people say! But since I've been recovering saddles I've learned that if I use the same leather on a B.72, and on a B.17 Standard, and on a Professional, and on a B.17 Narrow, the B.72 will be very soft, the B.17 Standard also pretty soft, the Professional a lot harder, and the Narrow very hard indeed. The width, not the thickness of the leather, determines the hardness.
So we confirm, that they have changed their recipe over the years.
I think I read that as well. But I may have made it up myself and posted it, and then you read it there. I do believe it, for sure.
Two things combine to give a saddle its stiffness - shape and material.
For sure, shape affects the compliance of the saddle. But for a given shape, thicker is stiffer. This is a simple fact of mechanical engineering.
I also have a tin of Proofide with the orange lid. My tin is from about 1973. When comparing new to old, you have to consider storage conditions and evaporative losses of the solvents over the several intervening decades.