Apple isn't big on admitting when it's made a mistake but it seems this one was too big to ignore.
It seems some iPhone 6s models from 2015 have an issue with their battery which causes the devices to power down unexpectedly. If you've had this kind of problem and your phone was made in September or October last year you should be able to get a new battery.
Affected users should bring their phones into an Apple store or an official reseller where they'll be checked to see if they fall into the right batch. Then the phone will get a new battery. It also seems that if you've paid for a new battery yourself, Apple might cover the costs of replacement.
This will be a minor annoyance for the company, and nothing like the scale of the double mass recall which Samsung faced with the Note 7 - a handset line which might never recover.