The Shulchan Aruch doesn’t mention shok b’isha erva, because, as the Bais Yosef explains (Orech Chaim 75), that is only an example of a makom mechusa that I might have thought was not considered a makom mechusa (the Bais Yosef cites the very Rashba you cited).
There is no indication that ???? ?????? fluctuates according to the generation/locale. See ?????, for example, who gives the face and hands as the counter example, rather than saying that it excludes a time or place where it is normal for it to be exposed.
Again, see ????? ????? and others who say such a thing is ???? ?? and has no affect on the halachah. This is not the Mishnah Berurah’s chiddush.