Defining and displaying erogenous zone can be arbitrary and it vary from culture to culture and over time. According to the book The Oxford Companion to the Body and Fashion and eroticism: ideals of feminine beauty from the Victorian era to the Jazz Age, Erogenous zones traditionally can be either adorned or tightly concealed. Psychoanalyst J. C. Flugel observed that "bare flesh is boring. Male curiosity is sustained by veiling the erotic site, by covering and exhibiting it at the same time. Heavy skirts concealed European women's legs for centuries, then legs lost their sexual allure when hemlines rose after World War I and the erogenous zone moved elsewhere." Fashion historian James Laver, who first discussed ‘shifting erogenous zones’ in the 1930's, used this concept to explain fashion or rapid changes in female dress. Influenced by psychoanalyst Flugel, Laver argued that women are born exhibitionists whose social subordination forces them to acquire male protection.

(It should be noted that for decades, historians accepted this version of costume history and believed that shifting erogenous zones fuelled fashion, but this theory is now challenged by historians, especially regarding modern fashion after the 1950's. Costume was a part of a broader visual culture and obeyed the same laws as painting or architecture. There are no doubt that there are strong underlying sexism to Laver and Flugel's theory and women dressed to please themselves as much as to please men. Costume historians observed that fashion always produces its opposite anti-fashion and menswear, appearances notwithstanding, conveys subtle but strong erotic messages. All these scholars agreed that sexuality was not the primary motive for dress. People dress to express themselves, to project their ideal self image, to display their political views, to signal their racial and national identity, and to assert their social position. In the new costume history, sexuality still shapes costume; dress is too close to the skin to avoid it. But the erogenous zone no longer dictates fashion nor serves as the only explanation for the human compulsion to decorate, ornament, and alter the body.)

The trope of not being allowed to show ankle is definitely a more modern concept of the past than anything else. Many styles over the centuries are ground level or just above, but that's simply the fashion. During those times women having skirts short enough to show ankles would be assumed to be working. It's just like the idea of "rolling up your sleeves" to get to work or "hike up your skirt" and get to it concept. During certain time periods, like the most part of the 18th century, skirt levels were high enough that it was a very normal thing to see ankles or even above. No reason to have such amazing shoes and cover them with skirts!

A colleague of mine was recently trying to find where this idea comes from specific to the 18th century since all imagery we have shows hems well off the ground. Turns out there was a woman, Countess de Genlis, who wrote a book about raising children to be proper religious, moral adults in the 1810s. She wrote about her earlier years where showing ankle was considered inappropriate, etc. However, it is abundantly clear how heavily skewed and opinionated her writing is. It was republished in English numerous times during the 19th century. I don't have the exact quote on me at the moment, but it has been referenced in history books as their source of the "no ankles" rule.