Matthew Panzarino, writing for TechCrunch following a statement from Apple:
Over the past couple of iPhone versions users have complained of “unexpected” shutdowns of their devices. Some iPhone 6, 6s, 6 Plus and 6s Plus devices could basically go dark unexpectedly, forcing a user to have to plug them into an outlet to get them to power back on.
Apple has been working on this very annoying bug and it says it has come up with a fix of sorts that should mitigate the problem on a majority of iPhone 6 and iPhone 6s devices. The fix is actually already on your iPhone if you have installed iOS 10.2.1 — something that around 50 percent of iOS users have already done. After letting the fix simmer on customer devices, Apple now has statistics to share on how it has improved the issue, citing 80 percent reduction on iPhone 6s and 70 percent reduction on iPhone 6 devices.
These unexpected shutdowns were a different issue than the iPhone 6s battery recall. From personal experience, I've heard quite a few friends mention how their iPhones' serial number weren't eligible for the battery replacement program despite frequent shutdowns. iOS 10.2.1 should fix that, and Apple is also rolling out a new message in the Settings app in case a device's battery has to be replaced.