Slate has the goods:
Naturally-occurring sour milk had in the mean time become increasingly rare, thanks to modern refrigeration. So commercial dairies, spotting an unfilled niche, began to culture it themselves, and sold the new product widely as buttermilk starting in the 1920s. This was much like the buttermilk we find in grocery stores today: Made from low-fat milk and lactic acid bacteria that grow best under moderate heat conditions. Dairies used low-fat milk because it was cheaper than whole milk, but still took on a thick, creamy body when cultured. Low-fat buttermilk also appealed to what Mendelson calls “a nascent fan club of dieters brought into existence—just at this time—by a new 1910s and 1920s cult of slenderness.”There is more history at the link, though strangely there is no discussion of pasteurization. Presumably it isn't available outside of New England but Kate's of Maine does in fact sell butter byproduct milk (as well as very good butter). I wouldn't be willing to bet that you can taste the difference between the real stuff and the cultured version in, say, a batch of biscuits... but it is possible to purchase in some places at least.