https://wccftech.com/iphone-8-plus-iphone-x-get-cpu-throttling-feature/

Apple had earlier admitted to throttling the performance of its iPhone lineup, and provide an affordable battery replacement for customers to ramp up that performance. The latest iOS 12.1 software update from Apple introduces a new change. Previously, the support page that explains the performance management feature said that iPhone 8, iPhone 8 Plus and iPhone X have an advanced hardware and software design which aims to provide an accurate estimation of power needs and battery’s power capability to optimize system performance. However, the page has now been updated after the release of iOS 12.1.

The page also says that the impact of the performance management feature might be less apparent on iPhone 8 and later devices. Stating the obvious, it also says that the rechargeable batteries in all iPhone models will eventually need to be replaced. The feature, which throttles the smartphone’s processor to stop the phone from abruptly shutting down as the battery degrades over time, is not there on the newest iPhone since their advance hardware update has rendered it useless.