It's water that builds up limescale that it's harmful for the carnivorous plants. The peat moss substrate that carnivorous plants like is acidic and the limescale neutralizes that.
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.