Does overcharging an iPhone degrade its life?
Solution 1:
An iPhone (in working order) stops charging when it reaches 100%, even if you leave it plugged in. Or are you perhaps asking about a malfunctioning iPhone, where for some reason this cut-off is not working?
Solution 2:
It's fine to leave the battery plugged in after the counter reads 100%. Apple's official iPhone Battery guide doesn't mention overcharging the lithium-ion in every iPhone.