A specific run of iPhone 6s models sold from September 2015 through October 2015 were manufactured with batteries that did not meet Apple’s specifications although the manufacturer certificied they did. They essentially did not last anywhere nearly as long as others would. Apple would replace those free of charge regardless of warranty on that model as long as it was in the effected serial number range which included those defective batteries. Those affected iPhones came from only one of three assembly plants that received those batteries from that supplier which lost the contract after that.
It was not the same problem as the shutdown caused by normal batteries becoming chemically depleted with normal cycle use as some are claiming to try and establish a “known problem” with this free battery replacement program as an earlier response to the same issue. These were actual defective, under-performing batteries that were not behaving as required in an identifiable subset of iPhones 6s units that once replaced with correct, new batteries did work correctly.