This was my experience too with a 6s. Even with a brand new battery I'd get maybe 3 hours of use out of my phone (and I'm a pretty light user) and furthermore it would often shut down registering as much as 30% remaining. I bit the bullet and upgraded to a new iPhone 11 a few months ago, and I now regularly end the day with 70% battery life, with no change to my phone habits.

So, at least anecdotally, there's more to the story here....