Was told the same thing, but my tap water works fine. If it’s true that it’s harmful I’d like to see some science (even citizen science.) Pretty sure it’s a myth.
Read literally any book about caring for them. They like acidic soil. Rain water is slightly acidic.
It might take a year or more to kill a plant by slowly draining its soil of acidity. Just like it can take a year to kill a big plant via inadequate lighting.