According to Apple keeping your iPhone fully charged at all times could cause more damage than good. If you are the type to charge your phone overnight this plays an even bigger role, which will result to chemically eroding the lithium- ion battery inside the phone. However; any iPhone running ios13 or later will conveniently have a function that analyzes how often the phone is being charged, this is called “Optimized Battery Charging” this function will also control the speed once the battery charge hits 80%.
Simply put, if you’re iPhone is at 100% or close to that at all times this may eventually affect your iPhone’s lifespan negatively. If you’re the “plug in at bedtime” type whenever you wake in the middle of the night & you see “fully charged” or “100%” unplug immediately.