This is absolutely a change. In the past it's been rare to need these kinds of facilities on a regular basis.
I think the most basic reason is that in the past the general view would be that if a child's toileting could not be managed within the regular facilities a school had, they are not ready for mainstream school. Now schools are viewed as a kind of childcare that needs to meet the needs of all students in the same setting, no matter how disparate.
So people who had children who were just slow developers might wait an extra year or so to start them in school. In fact at one time where I grew up (not the UK) there was a "school readiness" checklist, which suggested that a child should normally have met certain milestones before beginning school. Being toilet trained was one.
People also used to really try and make sure children approaching school age had met certain milestones. Able to do up a coat, comfortable in the toilet without help, know their phone number and last name, that kind of thing. People no longer seem to do this as often (I see kids of 9 quite regularly in my job who do not know their phone number, more than half in a class typically!)
Children with special needs were often in other kinds of provision if they couldn't manage with a fairly standard school washroom set up. Not always specifically because of that issue, but also because they often had other additional needs that the school wasn't the best place to meet.
As far as kids with no special needs - I think there are a few reasons more are not trained when coming to school. To my mind it is a combination of FT working parents, childcare facilities run without adequate support to really meet children's needs when they are there for long hours, some very bad advice recommending that parents delay training past the optimal time which makes it harder later, and a belief by parents that they should not be in conflict with their children and things like toilet training should come easily.