The discounted replacement price was part of Apple's apology to customers for not being upfront about an iOS power management feature that kicks in on devices with worn batteries, and the fact this feature can result in a slower experience.
While this power management helped customers avoid unexpected shutdowns, Apple's lack of transparency about the feature gave the perception it had slowed performance to encourage customers to upgrade.
The update targeted shutdowns occurring on the iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, and iPhone SE. The impact on performance was discovered after it extended the feature to the iPhone 7 and iPhone 7 Plus in iOS 11.2.
Apple's discounted battery swap offer is available to customers worldwide until December 2018.