Battery life span

It is neither good nor bad for the iPhone’s battery to use Low Power Mode. It does not affect Battery Health. It is designed to be used during circumstances when the battery’s charge level is low and the user needs the battery charge to last longer until the iPhone is connected to the charger. You can leave Low Power Mode all the time as well, however, it will affect the iPhone’s performance but your battery charge may last longer. In the end, if you are using Low Power Mode to maintain battery health, then don’t because it is not affecting battery health.

Axel F.

What is good for the iPhone is keeping it well charged.

Use Low Power Mode to save battery life on your iPhone - Apple Support

Theoretically it will reduce the amount of power used and how quickly the charge-discharge cycles accumulate. Whether or not the performance hit is important enough is up to the user.