DaMoshe: In the old days they had all these sorts of rules – in the public schools. It’s not that we have been building up chumros, but that the goyim (and that includes non-frum Jews) have become less tznius than in the past. We haven’t changed, but it is more noticable.
Today, if you see someone wearing a skullcap in public, you assume it is something unusual and probably religious (orthodox Jews, some Muslims, and some Christian clergy – but no one else). In the past in western countries skullcaps were common among all goyim, and back then no one thought that a yarmulke was a distinctly Jewish thing. It isn’t we adopted a chumra to wear a kippah – rather our style of head covering is no longer fashionable among the goyim.