https://www.theverge.com/circuitbreaker/2018/10/31/18047174/iphone-x-8-plus-performance-throttling-battery-management-ios-12-1-update

With iOS 12.1, Apple has brought its controversial “performance management feature” (aka, throttling) to the iPhone 8, 8 Plus, and iPhone X. This functionality dynamically throttles the phone’s processor as its battery degrades over time to stop the handset from randomly shutting down — it can be turned off if desired. Previously Apple has told US senators that “hardware updates” meant that the feature isn’t as necessary on its newest phones.

The company has always maintained that all of its products include “fundamental performance management” functionality to protect its components. A support page explaining the feature said the following about the iPhone 8 and later devices: