I'm having a hard time figuring out how to approach this, as there seems to be a big misunderstanding at the heart of the question. That is, "modesty 'before the decadent 20th century'" that relates to women's clothing is typically associated with Victorian culture just before the decadent 20th century, rather than the Middle Ages.

There is some discussion to be had regarding women's clothing and modesty in the Late Middle Ages. Broadly speaking, this is when clothing began to be fitted closely to the body, and its prestige began to relate to the cut based on what was in fashion, rather than simply how expensive the fabric was and how much of it was used - and as a result, women (and men) could show more of their bodies than their hands, heads, and feet. For instance, in French romances of the 11th century, seductive women would be described as wearing unlined silk bliauts (very tight-fitting long gowns) without chemises, their skin peeping through where the gown is laced at the sides and on the sleeves. Fashionable necklines could be wide, or low, or wide and low. Busts could be bound with bands to be high and firm, or allowed to be more curvaceous. And obviously, these offered opportunities for people to critique women for being too vain, frivolous, and sexy. But as far as the kind of "modest women don't show their ankles, or their shoulders during the day" thinking you're implying goes, that's a later development. A fashionable woman wore a long skirt while a working one who needed to walk with her hands busy wore a shorter one, making length entirely a practical consideration; and since nobody showed their forearms, let alone their shoulders, there was no value in setting up a dichotomy of good/bad behavior there.

The attitude you describe was largely characteristic of the 19th century, particularly the later half. By the beginning of Queen Victoria's reign in the UK, 1837, it was a well-established principle that the fashionable could wear "evening dress" or "ball dress" that featured quite open necklines and short sleeves, in opposition to the day dress worn by the fashionable and unfashionable alike, which had long sleeves and was becoming more covered up at the neck. (Around 1837, the gown was more typically made with an open neckline that was covered or filled in with an accessory, but by the late 1840s bodices were simply being made with smaller head openings.) Moralists who were opposed to bare shoulders and necks in general took issue with this as being both unbecoming and far too naked, and a second concern building off of that was that women who were not wealthy wives at fancy dinners were copying the style - which some saw as bad because the hardy, unrefined, and in some way "purer" working classes were being corrupted by decadence, and others saw as bad because it made class distinctions too difficult to make out. (As I wrote in this previous answer, fine clothes were seen as something that prostitutes wore inappropriately, because it was above most of their stations - even when they were able to afford secondhand evening dress, they didn't have the right to wear it and it was being inappropriately worn in the street.) For example, you can read the satirical "Shoulder-straps' Petition" in The Christian Lady's Magazine, 1841, which highlights the above issues and adds a plea for health.

As for ankles: while the idea that the Victorians were totally afraid of the lower leg being seen is a gross exaggeration bordering on a myth, for the last ~60% of the century fashionable women's dress did go roughly to the floor, and even un-affluent women are generally portrayed in genre images as conforming to a similar standard. The "wealthy woman, long skirt; working woman, far from the floor" standard lasted in England and France for a very long time, with skirts that showed the feet and even ankles not being common in fashionable dress in both countries until the late 1810s (particularly in ball dress) - and by 1840, they were back to it. During the Victorian period, a short skirt that put at least some of the legs on display was part of a dancer's outfit, whether a dancer in the romantic tutu of La Sylphide or a burlesque performer (Mashable has a great collection of costume photos here), and it's particularly this association that made the idea of short skirts taboo. When skirts were worn shorter for hiking or, later, bicycling, there was little that was scandalous about them. Queen Victoria even sparked a fashion in the 1860s for a "Balmoral petticoat", shorter than an ordinary skirt and worn when for activities like walking across the countryside or skating.

If I've misunderstood you at all, feel free to ask follow-up questions!