This website makes some claims about what has caused both growth and decline that are pretty contentious. I think that many would argue that the growth following the second world war was the result of massive state investment in public services like creating the NHS and the building of council housing. They baffelingly attribute yhat growth to the Conservative Govrernment of the 1930s rather than the post war labour…
Water quality by country. https://worldpopulationreview.com/country-rankings/water-qua... US is good while countries with more left leaning setups do bad. It is not as black and white as you imply.
It has always been either over chlorinated, or out of a well with all sorts of contaminants such as sulphur.