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.