The thing with bad parenting, you need more than that to have an able child still in nappies years after they should be.
First, no other carers or childcare settings, because childcare settings particularly will always encourage and support toilet training, with parental support or not.
Secondly, a child that had no will,interest or curiosity to be potty trained. A child that doesn't care about fitting in , or a sense of embarrassment, or wanting to be part of the routine and doing what everyone else is doing. You might find plenty of those at 2, but the numbers dwindle at 3,4 and minimal if any, at 5. It might take a while, but once in an "official " setting it all starts.
So how many kids does that actually leave?
As an aside, we have plenty of kids with horrific home lives, kids under SS , kids in care etc. none of them are/were in nappies unless there were other issues at play as well, or they were literally too young.