Not the person you were asking, but to me, "suburban" is not defined by density, but by how walkable an area is. Even relatively low density neighborhoods can be urban under this definition as long as residential isn't zoned far away from commercial districts and everything is within a few minutes of walking or an easy bus ride. Suburban is when having a car provides an enormous QoL boost, but isn't strictly necessary. And rural is when a car is absolutely required to survive.
These are fairly car centric definitions. But considering the car enabled the suburb in the first place, I don't think it's necessarily wrong to use it as the metric.
But I think the modern suburbs (in the USA) were more of a "white flight" phenomenon, enabled by cheaper construction methods and perhaps a confluence of a few other post-war trends/policies.
White flight swelled the population of suburbs but they were already substantial. I think it would be a misleading picture if you just choose one point in a continuous process and start from there.
We don't need to create the universe to bake an apple pie from scratch. I agree that a history of the suburbs might begin before WWII, but the general concept of needing to start at the beginning is a slippery slope. I'd say nearly all histories must pick an arbitrary start, mid-stream.
I think there is still, even now, a difference between these older, inner-ring suburbs with many people commuting to work by train (and generally smaller plots and the like) and newer outer-ring suburbs or exurbs (or just rings around newer cities) which are more car-focused from the start.
These are fairly car centric definitions. But considering the car enabled the suburb in the first place, I don't think it's necessarily wrong to use it as the metric.