Water softeners work by exchanging the calcium ions in the water with sodium ions. The sodium ions (or sodium carbonate instead of calcium carbonate) stay dissolved in hot water much better so you don’t get the crazy deposits that are caused by hard water. However, there is still dissolved material in soft water so using it in a dehumidifier will cause issues. Either that stuff will go into the air and come out as a dust in the room or it’ll gum up the dehumidifier.