https://www.cnet.com/news/apples-battery-throttling-comes-to-iphone-x-8-and-8-plus-with-ios-12-1/

In iOS 11.3, Apple gave iPhone users access to its battery management tools, which let people using older iPhones turn on or off the throttling feature (called peak performance capability). In a support page updated Wednesday, Apple said that the feature will come to the iPhone 8, 8 Plus and X with iOS 12.1. But Apple points out that performance management may be less noticeable due to the newer iPhones' more advanced hardware and software.

iPhone batteries naturally degrade as time goes on, and the throttling and unexpected shutdowns are more likely to occur with older iPhones. Since the 8, 8 Plus and X are newer, their batteries are likely to be in better shape than their older iPhone counterparts.