Visited with my son-in-law today about the iPhone battery issue. He's a professor of electrical engineering at a large state university.
My understanding of his explanation (any errors are mine) is that while the iPhone lithium-ion battery specs at a nominal 3.8 volts output, it is subject to Ohm's Law, in that its internal resistance causes a voltage drop proportional to the resistance. As the battery discharges, the internal resistance increases, reducing output voltage. The iPhone circuitry is shutting off the phone when the output voltage falls below a nominal value, as any phone would do.
As the battery ages (increased number of charge cycles), the internal resistance builds faster during discharge than it did when new, so the voltage drops to that minimum nominal value sooner. (So the phone shuts down sooner with an older battery.) But not only does that older battery drop to its nominal minimum voltage sooner, the fall-off curve is steeper than it would be for a new battery. Hence, while the battery output voltage for an older battery may show, let's say, 2.8 volts at 30% battery remaining, that voltage may be dropping so fast that it reaches the threshhold quickly afterwards and shuts down, whereas a new battery would continue to decay past that 30%/2.8 volts more gracefully.
The problem can be solved with:
1) more frequent battery changes, or
2) a battery with a slower internal resistance build-up, or
3) a battery that takes more charge cycles before the sharp voltage declines begin, or
4) phone circuitry that requires less voltage before shutting down or that demands less current overall, or
5) slowing the phone down so the battery take longer to discharge.
It appears Apple took option 5) in iOS 10.2.1, since the other options entail replacing the battery or redesigning the phone. Going forward, until Apple and its battery suppliers can resolve options 2, 3, and/or 4, users will be left with periodic battery changes (1), or run the risk of (5) software throttling.
On a related note, his iPhone 6s was one of the recalled phones, so he had his battery replaced six months ago under the recall program. This new battery is already causing shutdowns at 30% or more remaining. Says he's not buying another iPhone until Apple either changes battery suppliers or the specs the suppliers build to.