Apple has determined that a very small number of iPhone 6s devices may unexpectedly shut down. This is not a safety issue and only affects devices within a limited serial number range that were manufactured between September and October 2015.
If you have experienced this issue, please visit an Apple Retail Store or an Apple Authorized Service Provider and have your device’s serial number checked to confirm eligibility for a battery replacement, free of charge.
My launch-day iPhone 6S was affected by this; I had it swapped early this year. If you’re seeing this issue with your iPhone — even if it’s not within the correct serial number range — be sure to head to your local Apple Store to inquire about a replacement.
Update: I just checked my replacement iPhone and — funny enough — it, too, is eligible for a battery swap according to 9to5Mac’s list of serial numbers.