Daas,
It is meduyak in the Gemara that Daas Yehudis cannot be D’Oraisa (unless you say the Gemara is lav davka, which is dochek). The gemara quotes the Mishna that says going out with uncovered hair is Daas Yehudis and the Gemara asks “uncovered hair, that is
D’oraisa?” The clear implication is that it therefore cannot be Daas Yehudis.
I used arms as my example along this debate because it was the only part of the body I was certain was only Daas Yehudis. However I have never seen any source for an chiyuv D’Oraisa for a woman to cover up body parts not during tefillah, and while I can’t prove it, I am pretty sure it is only d’rabbanan or Daas Yehudis.
The bathhouses in those days were public places and therefore the requirement to cover hair should apply, even if it there are only women around (unlike baring arms, there is no indication in the Gemara that hair covering has anything to do with men being around). The Gemara is clear however that the chiyuv to cover hair does not apply in the home.
“R’ Moshe and others clearly write that arms are assur despite any prevalent mode of dress.”
R’Moshe is discussing saying Krias Shema, he is not saying that in a frum society where women commonly go out with bare arms, it would still be ossur to walk around like that. Pashtus, Daas Yehudis a minhag and if the minhag changes, the minhag changes.