In Europe, it’s true that there were many women who didn’t cover their hair. That was another mitzva that was overlooked there, along with shatnez and some others. That doesn’t mean it was ok.
I heard from a reliable source that when R’ Aharon Kotler came to the US, his wife didn’t cover her hair either. She started doing it after they moved to Lakewood, when R’ Aharon asked her to.
There were many things that were overlooked in past generations. Our observance of mitzvos has grown.