I doubt most freshwater sources have enough minerals in them to not be corrosive enough to need to add anything to reduce corrosiveness (ground water wells, rivers, etc...). The only purpose for adding fluoride would be to add fluoride for dental purposes.

The only thing I could think of that might have corrosive issues would be desalination water that was overly purified like the sibling comment mentioned. But even then, they should be adding some kind of minerals back before distributing it to people's homes. Ultrapure water tastes horrible and likely isn't safe for drinking long term.