Yes! It's the equivalent of distilled water, being very pure. Your plants will love it, and it's a good way to save water.

Not necessarily pure all the time. If your tap water supply has chemicals or solids in it(even a little), those solids and chemicals can build up in the de-humidifier over a period of time and then peel of the inside all at once, giving your plants an overdose of those substances.

If there is no humidity(moisture), why is there a de-humidifier? When someone turns on the cold water faucet a little water evaporates- to be picked up by the de-humidifier. When the hot water faucet is turned on much water is picked up by the de-humidifier. When someone takes a bath or shower MUCH water evaporates, to be picked up by the de-humidifier. When the refrigerator or freezer door is opened the moisture in the very cold air evaporates( cold air from a freezer evaporates as fast as hotwater). When lawn sprinklers are used on a warm or hot day, some water evaporates- some of that gets into the house. On a cold day water sticks to windows and then transters to the inside(except double pane glass). Leaving the toilet lid up allows a way for some of the water in the toilet bowl to evaporate up to where the dehumidifier can pick it up. If the water is not absolutely pure, some of the solids will get through the de-humidifier and/or stick to the inside of the dehumidifier and come out later all at one time(peel). At that point thesolids are ata bunch higher proportion than usual. If anything there probably would only only be a discoloration of the leaves, but the plants may die if the solids build up in the soil they use for nourishment.