Are the majority of municipal water supplies of drinking water safe enough for long-term use?

Safe enough is a vague description. We would say none can be trusted, simply looking at the averages and knowing the primary scientific and engineering standards are corrupt and serving the Extraterrestrial Alliance in the details in how water is treated and not treated. So there is a trade-off. Much water of uncertain origin will be present in prepared foods, so it is not only drinking water that contributes to ingestion by humans themselves, but to the extent humans can use a strategy to reduce contaminants in what they take in, this would be highly advisable.