I don't know about you, but I've taken my fair share of art history courses, which tells a lot about what people were wearing over the years
What many Christian conservatives call "immodest" even today were worn by the poor and the working class in Biblical times, yet it is not mentioned for them to cover what. What is mentioned? To be modest of price, of flashyness, to not make others feel bad about themselves by flaunting wealth and status. Modesty in clothing is not about what you are covering, but how you present yourself no matter what you are wearing. A truly modest person can be modest in just underwear. A truly immodest person can be immodest in a huge oversized, full-length, full-sleeved robe.