Apple has determined that certain iPhone 6s and iPhone 6s Plus devices may not power on due to a component that may fail. This issue only affects devices within a limited serial number range that were manufactured between October 2018 to August 2019.
This is from the page that was just put up on Apple’s support site.
Apple also provides a refund if your iPhone 6s or iPhone 6s Plus was affected by this issue, and you paid to repair your device.
The program only covers devices for up to 2 years after they were sold.
To see if your phone is eligible for this program, visit the support website and enter your iPhone 6s or iPhone 6s Plus’ serial number.