A reminder of Valve’s FAQ, given recent article on Websites trying to contradict (e.g. https://www.slashgear.com/1314340/steam-deck-plugged-in-overnight/):
If your device has been left on the charger for long periods of time, it may report less than 100% charge. This is normal. We allow the battery to slowly discharge after long periods of time under charge to optimize long-term battery health.
It would take longer than that for charging system problems to become apparent. But Valve seems to have attention to detail on things that could ruin their device.