When mine were little, the pre-school (took children from 2.5) didn't take children in nappies.
So from the age of 2, every mum in the village worked like demons on potty training. Mum and toddler groups were full of children sitting on potties, we all shared tips on how to do it by the magic 2.5 date, and there was a real incentive to keep working at it when things got difficult. DS hadn't read the books, showed no signs, didn't want to use the potty, BUT I took him up regularly to see his slightly older friends playing at pre-school and it gave both of us the added push to work on it.
The vast majority of these children went to pre-school on time. DS was a little late. Children with SN went in nappies, that was fine.
Just after DS started, taking on board legal advice best practice from ERICetc, the pre-school removed the requirement for toilet training. It's over 12 years on from that now, and apparently it is now more common than not for children to be in nappies for the very large majority of their time at pre-school, some not being trained by the time that they start Reception. What has been lost has been the shared knowledge, the support to 'keep at it when it's hard', and a key deadline. What has been gained is less pressure on children, especially on those children who may have special reasons why they are difficult to toilet train (DS had toddler diarrhea for 18 months, almost up to 2.5 years, which was one of the reasons why training him was difficult).
I do think that the pendulum has swung from 'parent led' toilet training to ;'child sign led' toilet training, and that has moved the average age later but perhaps the process easier. am sure that this has made lives easier and less pressured - but it does mean that some children aren't toilet trained who could be IYSWIM?