As stated in the other comment, old batteries often physically couldn't handle the voltages required to run at peak performance, so if iPhone detected these con

As stated in the other comment, old batteries often physically couldn't handle the voltages required to run at peak performance, so if iPhone detected these conditions, it would temper those peaks so that the phones would stop randomly turning off for people. The issue was that they didn't tell affected device owners that this was happening, and now they do in Settings.