I do not think they were under reported just under talked about. There used to be what was considered a decent reticence about discussing health problems.
Cancer was seen as a death sentence, so not discussed, acid reflus, was called indigestion and dealt with, often unsuccessfully with Rennies and equivalents.
There was certainly much less Type 2 diabetes. We know that. Dementia was a bit like indigestion some old people went a bit doolally and were looked after in the family. Of course, a shorter lifespan meant that many people died before they suffered from it.
The other thing is that there are now remedies for so many of these diseases. There are far more effective drugs for acid reflux, thyroid problems.
In 1950 life expectancy was 69. In 2025 it is 82. That is an increase of 23 years. However part of this is accounted for by the fall in infant mortality from 32 per 1,000 births in 1950 to 2 in 2025