I had never heard of calling that footwear "thongs" until I went on message boards. I wouldn't call them that anyway, since thong=underwear (or rather, pretend underwear) so it's confusing.
Actually, in the 18th century, women went bare bottom. It's the 19th century that bloomers came into fashion (and had crotch slits). Prior to that women wore chemises that came down over their hips under their outer clothing.