I learned recently that some women don’t have their milk supply appear until weeks after birth and some never get sufficient supply at all. What happened to infants of such women before baby formula? Did they just die? What tactics did women use in these situations to keep their babies alive?
There were many folk remedies to help let down milk, and some of the herbs they use did work in some cases.
If they could, they would have other women nurse the babies, either relatives or women of poorer social class who could use the money.
"Wet nurses' were common up to the invention of formula, and many slave women often acted as wet nurses in the 19th century.
You may enjoy reading this article: "A History of Infant Feeding"
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2684040/
They used animal's milk at least 2000 BC and probably well before, as needed. "The type of animal's milk used was dependent on the kind of animal that was available—goats, sheep, donkeys, camels, pigs, or horses. The most common and universally used milk for artificial feeding, however, was cow's milk"
When they analyzed animal milks relative to human milk, that is when formula was born because they realized animal milk is not exactly right for human babies by itself.
But in the past they would have done what they could to save babies, not knowing germ theory or the difference between the milks, and fed the baby best as they could and many survived.
Indeed, Roman myth is that two twins (Romulus and Remus) were suckled by a she-wolf; Greek myth mentions heroes and gods suckling animals, the Talmud says babies may suckle from animals if their health demands it, and so on.
I have recently found answers by /u/mimicofmodes and /u/Superplaner and /u/amifufu for What happened when moms couldn't produce enough milk before formula? but others may have more to say.