Using Low Power mode all day effect battery health or not?

It will make your battery last longer, as it won’t need to be charged as often. So indirectly it will preserve your battery health. But you are also giving up a number of features that are turned off in low power mode.

As I said, using low power mode will improve battery health because it will reduce the number of full charge cycles needed to keep the phone running. And battery health is almost exclusively proportional to the number of full charge cycles.