Old-school buttermilk is the milk left after churning butter and is not today's 'cultured buttermilk'.
Buttermilk was what was left after the soured milk had been churned and the butter removed. There were always small particles of butter left in it
For years I had assumed that skim milk was the best substitute (as it's milk with the fat removed), but this suggests that it's both soured and has a little bit of fat left (but not even close to homogenized).
Is there something roughly equivalent available today, or something that I can make to approximate old-school buttermilk without churning my own butter**?
** It also hints that today's 'sweet cream' butter is not the same as butter in the old days. I don't know if 'cultured butter' might be closer, or a blend of cultured & sweet cream butters.
Clarification: I am not looking for a replacement for modern 'cultured buttermilk'. I'm quite aware of the substitution for today's buttermilk when baking of using milk plus an acidic liquid, or of thinning yogurt. It's possible that this is also a good substitute for historical buttermilk; if so, please acknowledge in your answer that you're aware that they're different. If you've spent time on a dairy farm, please let us know if the dairy was using fresh or soured milk for their butter (because everything that I've found said that it was made with soured milk historically).
From The Settlement Cookbook (1945), in the discussion of dairy products (pp. 45-56):
Cream is the fat that rises to the top of the milk if left standing. For Whipping Cream, see page 498. Skim Milk is the milk left over after cream has been skimmed off. Buttermilk is the liquid left over after cream is churned into butter. ... Cultured Sour Cream and Buttermilk may be obtained from most milk dealers.
In this case, it doesn't specifically mention that soured milk is used, but I've seen other references from the late 1800s to early 1900s that said that butter was made from soured milk. (and one of them would occasionally say 'new milk' instead of 'milk', suggesting that their use of 'milk' was sour milk or buttermilk. I suspect that before refrigeration, it's quite possible that milk left to separate into cream would sour by the time it was finished. They do mention in that same section:
Sour Milk is valuable in cooking and may be obtained by keeping milk (preferably raw milk) undisturbed in a shallow covered pan at a temperature of 90° to 100°F. until it becomes thick and clabbered. If it sours too slowly it becomes bitter.