PSA: Steam Deck *does* allow a slow discharge, and the battery life is safe being left on the charger (e.g. overnight)
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Steam Support :: Steam Deck - Basic Use & Troubleshooting Guide
https://help.steampowered.com/en/faqs/view/69E3-14AF-9764-4C28
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.