In dealing with issues relating to Tzeni'uth (modesty) and clothing, a lot depends on "local" standards of dress. For example, the RaMBa"M (here) explains that in places where women do not normally go out with just their hair covered, but with veils as well, they should be sure to wear veils as a matter of Halachah. However, since any deviation in the local standard would be a violation of this principle, it would stand to reason that we ought never to have changed our standard of dress from the time of Sinai. Since that is observably not the case, does this mean that, both in practice and in Halachah, we recognize societal standards in determining how we ought to dress in terms of modesty?
The Perisha (YD 182:5) gives two suggestions for how a place could change the associated gender of a particular clothing or action: either we follow the custom of the local non-Jews or a whole community of Jews could decide to change together.
Modesty norms, which are all about not attracting attention and hence based on the current facts on the ground, would seem to be no worse.
Rabbi Broyde has a yutorah mp3 regarding hair covering where he says it appears that the general standard of erva is probably subjectively tied to what's considered appropriate by modest non-Jewish women in general society.
Societal standards definitely affect how we ought to dress. Rabbi Yehudah Henkin quotes his grandfather, who said, “there is no prohibition against women wearing loose pants." If the purpose of tsnius is to act in a way that does not draw unnecessary attention to oneself, the way other people dress certainly makes a difference. What was necessary to achieve this level at one time is different from what is necessary now.