Think Madame X. 1870s and 1880s evening dresses. It is usually told that armpit shaving only started/came into vogue in the 1920s, with the advent of sleeveless flapper dresses (and the spread/intensification of advertising). However, there was a fashion of sleeveless evening gowns back in the 19th century, some 50-40 years prior. What was the standard of grooming back then? Thanks :)
It's hard to say, as this was not a subject that was publicly discussed. Magazine advertisements offered creams to removed unwanted hair, but do not specify what hair would be unwanted - just facial hair? Or more? While some 19th century nudes have armpit hair, others don't. The erotic story My Secret Life mentions armpit hair only one time, when the protagonist is talking to a friend in amazement after finding out that women had it.
The only evidence that underarms only started to be depilated in the 1910s is that the first women's razor appears to have begun to be marketed then, which is not definitive proof at all. The sad answer is that we don't really know.