I realize today that most Chinese women don't change their names after they get married. I also know that during Imperial times, they did change their names (and vestiges of this practice still exist today).
I'd like to know the timeline of this shift from a maiden-name/husband's name tradition to the one that exists today, where the woman doesn't change her name. Also, I'd like to know what the exact situation is for places like Taiwan, HK, Macau, Singapore, etc., because a lot of Taiwanese and Hong Kong women I know don't change their names either. Did this spread from the PRC? When did it spread?
This is not correct. The cultural tradition for most of Chinese history was that the woman did not take her husband's surname, and this is mostly the same today except for some exceptions in Hong Kong or overseas Chinese communities. In fact, Hong Kong is one of the few places where Chinese women can take their husbands' surnames, but this is not the necessary rule. For example, in Hong Kong the politician Carrie Lam, or Lam Cheng Yuet-ngor (林鄭月娥), was born with the surname Cheng (鄭) but married a person with the surname Lam (林). It's similar with the politician Regina Ip, or Ip Lau Suk-yee (葉劉淑儀), born with Lau (劉) but married to an Ip (葉).
Let's have a hypothetical woman, He Ying (何英). Last name is He, first name is Ying. Let's say she marries a man, Gao Qiang (高強). She might then become known as Gao Taitai (高太太) or Gao Furen (夫人) in some Chinese communities, which will mean something like "Mrs. Gao." But those terms are titles and the woman in question didn't change her name. Gao Taitai and Gao Furen would be better translated as "Wife of Gao." So this hypothetical lady should be thought of as "He Ying, Gao's wife" and not "Gao Ying, Gao's wife." She's still He Ying (何英), not Gao Ying (高英). And this thing about Taitai/Furen is widespread in only certain Chinese communities. In Mainland China a married woman would probably just go by her last name.
Here's a timeline for Taiwan.
In general, the change in Taiwan was simply a part of the wider women's rights movement, which in turn was a part of the even bigger gender equality movement.
TL;DR: before 1995, women need to be prepared to piss off their in-laws to keep their surname. After 1995, the law became neutral and no longer default to women taking on their husband's surname.