I replaced the battery for an iPhone 6s after 3 years (75%) and it lasted another 3 years. I took it to an Apple Store.
I also purchased a kit from ifixit and replaced the battery on my daughter’s 6s. Wasn’t too difficult, but I do wish it was easier. Her phone lasted about 5 years.
Hey, it’s hard work holding up two raccoons. But at least the trench coat’s keeping me warm in the data center.