In a statement provided to TechCrunch, Apple has indicated that improvements made in iOS 10.2.1 have addressed most of the problems with unexpected shutdowns that some iPhone 6 and iPhone 6s users were experiencing. Apple notes that approximately 50 percent of iOS devices are already using iOS 10.2.1 and that Apple has been receiving diagnostic data from users on that version revealing a reduction of over 80 percent on iPhone 6s devices, and over 70 percent on iPhone 6 devices. Apple notes that in the unlikely event users are still experiencing unexpected shutdowns, with iOS 10.2.1 the iPhone can now restart without needing to be connected to power. In its statement, Apple emphasizes that these shutdowns are not a safety issue, however the company naturally wants to mitigate the inconvenience of the issue for its users. iPhone 7 models are not affected by the issue.
The TechCrunch report goes on to explain that the issue was likely due to suddenly high spikes of battery usage causing older batteries to delivery power inconsistently, causing an emergency shutdown of the device. New batteries were unaffected as they provide more stable power, but as batteries are placed through more charge cycles, they become more susceptible to these types of spikes. iOS 10.2.1 presumably tweaks the power management system to significantly reduce the incidents of shutdown, but they clearly haven’t yet been able to completely eliminate them.