I thought the flappers wore the buckles of their shoes undone, thus the sobriquet “flapper”?
I recall that “flappers” were called that because they didn’t wear garters to hold their stockings up; therefore their hose were “flapping.”
This is borne out by the contemporaneous song, “Five-foot-two,” which includes the line,
“Turned-up nose, turned-down hose. Flapper, yes-sir, one of those ... has anybody seen my gal?”