Rates of chronic constipation are also rising which can (in addition to a variety of medical reasons) be triggered by poor diets lacking in fibre. This can interfere with bladder and bowel control as well as causing impaction leakage and stress around toileting because of the pain and discomfort.
Having had a child whose autism wasn't diagnosed until 9, I'd say that in the absence of clear medical or developmemtal needs, it is lazy to not have tried toilet training before starting school, but success is a different matter.
In our case DS1 had no interest at all when we tried at 2½ in the summer. We used washable nappies so it wasn't the dry disposable feel being a factor. His communication skills were also just above the threshold for intervention which didn't help at that point in time.
The advice of the time (10+ years ago) was to look for the signs that the child was ready and make the most of summer for nappy-free time. He was a winter baby and just wasn't ready in that summer window. Nursery supported when he moved to the pre-school room at 3 (during the winter) and trying together those few months later did help. His autistic traits didn't present together strongly enough to begin the diagnostic pathway until he was 7-9.
It was great having DS2 who was self motivated to train by 2½ (although he wasn't dry at night until 6) however he had more urgency for toileting for many years.
In the 80s I had a "potty chair" so presumably I was expected to sit on that for a longer period until something happened. "Accidents" in young children were common into the infant school years so I'm not convinced that perceptions of what classed as "toilet trained and successful outcomes were exactly the same, but disposable nappies were new, expensive and much bulkier so weren't attractive options for prolonged use as a strategy to avoid "accidents"
"Hydration" culture didn't kick in until the late 90s-early 2000s so access to drink will have been a drink at breakfast time, access to a fountain for a couple of gulps of water (in the toilets at my school), a small (200ml?) cup of water at lunchtime. No constant swigging from large bottles through the day. It's another layer of change that will affect frequency of toileting demand.