It's not a bug or issue with those phones, it's how batteries behave -- over time, they lose both their capacity and the power they output. Apple decided to throttle their phones via software instead of letting them crash.

I've said this before, but it was the right idea executed the wrong way. iPhones give you a warning when they overheat, and this throttling should have gotten a similar warning with a link to an FAQ explaining the battery dynamics.