While is theoretically correct that it's not healthy for the battery to stay at 4.2v constantly, the Switch suffers from a similar fate while being in the dock all the time, yet, Nintendo confirms that it doesn't really harm the battery.The gamepad is always on. The best thing to do is add a hardware power switch, as it will drain in a short amount of time even when the Wii U is not used. Which will damage the battery over time. Leaving it on the charger 24/7 will also damage the battery over time. Basically Nintendo designed this to have the battery self destruct no matter what you do. A hardware power switch is almost mandatory.
My Wii U GamePad battery took around seven years of heavy use to die (the 1500mAh one), and I'm still waiting for my Switch battery (after other seven years of being mainly docked) to lose enough capacity to call for a replacement. Even if not the most optimal conditions for the battery, it's nothing to be really concerned.
However, it's still recommended to store batteries at 40% (≈3.7v) if storing for long periods of time.