The iPhone 6s and iPhone 6s Plus were announced four years ago, in 2015, but Apple continued manufacturing them even this year, as it introduced the iPhone 11 line-up, until August, when the models got discontinued. The Cupertino-based company has determined that, due to a component that may fail, some iPhone 6s and 6s Plus models may not power on. This issue is limited to a small amount of devices which were manufactured between October 2018 and August 2019.
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 — Apple
You can check if your phone is affected by going to Apple’s support page in the source link below and checking the serial number of your device. If it matches the ones in the affected iPhone 6s or iPhone 6s Plus batches and it is eligible for repair, Apple will do that free of charge.
The iPhone-maker doesn’t specify what component specifically creates the issue, but as long as it is fixing phones for free, it shouldn’t matter much, to the user, at least.