She doesn't just show "her lower legs and shoes", she shows almost the entirety of her legs, including a significant portion of her bloomers:

So yes, it is about modesty, but the maids don't just get upset because she's showing a bit of ankle, she's basically flashing her underwear at them.

While singing a pro-suffrage protest song, no less.

Times have changed.

In Edwardian times the glimpse of a woman's ankle or stocking was considered shocking and verging on pornographic.

TV Tropes explains..

In olden days a glimpse of stocking

Was looked on as something shocking...

—"Anything Goes"

If a show has an episode set in the 1800s or early 1900s, or is referring to that era, expect mention to be made of The Ankle Taboo. This is often played for laughs; a picture of a bare ankle will be referred to as pornography, and the sight of one will induce much shock, scandal and monocle-popping. Other times, "can't show your ankles" will simply be used as short-hand to sum up all of the repressiveness of that era. Note that a lot of things we tolerate today were severely frowned upon in those days (and vice versa), but the exposed ankle is probably one of the easiest, quickest, and (in today's culture) silliest examples of Values Dissonance that a show can portray, and thus works best for a quick gag

The setting was of a far more modest time - these were things just not done in polite society - as stated in other answers.

But also consider that this was made in a more "relaxed" 1964 (mini skirt era), looking back on the Edwardian period, and doing so in a light comedic tone. Not only would Mrs. Banks' actions be unlikely for anyone in the actual setting, but the maids' reactions in the movie would have been exaggerated for comedic value.