This is almost exclusively a Millenial/Homelander thing (with Boomer or Gen-X parents). My mom took the elevator train in from Jackson Heights to Manhattan every day for school from the time she turned 12. That's a 50 minute train ride with 2 connections. This was in the early 60s.
I feel like there's basically a two-generation swath where kids were treated like they'd break if you breathed on them, which coincides with the first generation that's never known real deprivation (Boomers) raising kids in an environment where their leaders are trying their hardest to create an environment of fear to maintain social control in the absence of war and real deprivation. It's slowly reversing itself as Millenials are themselves having kids and realizing all the ways that they were screwed over by not being forced to face risk & adversity in their formative years. (There's a big selection bias, too, in that Millenials who buy into the culture of fear are just not having kids because they think the world is hopeless, meaning everyone who has a next generation tends towards the un-anxious population.)
From my reading of your link, it looks like the incident that brought her fame/infamy happened in 2008. That's right during America's obsession with helicopter parenting, so the timeline makes sense.
If you go back to the 1980s, what she did wouldn't have been that remarkable. In that time, kids like me were gone from home for hours, walking or riding bikes for miles away from home, and we didn't have cellphones for our parents to keep tabs on us. Statistics show that crime rates were much higher (they peaked in the mid-70s, and have been falling since); the difference is that Americans just weren't paranoid back then the way they are now.
The problem, of course, is that in America a driver will kill your four year old, and it will somehow be your fault for letting your kid walk to the metro station.
Middle school kids taking public transit by themselves is pretty normal.