Ok, I'm curious...
How much do you use your phone during the day and what kind of charging do you do?
From what I understand about iPhone batteries, to keep the battery from discharging during use, it would need to be plugged in to use mains power. If you're using it unplugged, you're going through cycles and that should deplete your battery health?
But then, I see this... I had my battery replaced about 18 months ago and I'm back at 80%. I've been at 80%. Battery health is excellent though. My aunt's iPhone 8 is at 83% but needs servicing. She's had that for maybe 4 years.