It started in the 80s, related to the Satanic Panic, particularly the McMartin preschool trials.
Before that, it was expected that even very young children could be outside alone even in urban settings.
Don't take this for granted. I suspect this behavior is actually caused by an inherent anxiety in some subset of the population. When they don't have real (and likely) dangers to worry about, they will find some kind of tiny risk and overamplify it as something to focus their anxiety on.
Removing all things dangerous from the environment will only serve to amplify their tendency to do so. A mum in a high risk high crime neighbourhood is probably more likely to let her kids roam free (and less worried) than mums in upper-middle-class neighbourhoods where there is virtually zero crime.
My hypothesis is that the solution is to let children experience activities that are moderately dangerous (ideally through risk of pain, minor injury or some social stress, with risk of death or permanent injury kept minimal). This helps (I think) callibrate their ability to estimate risk as they grow up.
This would allow them to ignore imaginary risks like the one discussed here, and may help them identify situations that come with real danger.