In 2017, Apple was at the center of controversy after it was discovered the company was throttling older iPhones. Apple explained that it did so in an effort to protect devices from unexpectedly shutting down as their batteries degraded.
The controversy eventually lead to Apple introducing a Battery Health feature in iOS, providing users with more transparent information about their device and its performance.
We thought the controversy was behind us, but the same issues are brewing once again. According to Apple's support page, the arrival of iOS 12.1 may throttle the performance of devices Apple released at the end of last year.
Apple previously said the iPhone 8, iPhone 8 Plus, and iPhone X featured hardware updates that would "allow a more advanced performance management system that more precisely allows iOS to anticipate and avoid an unexpected shutdown."
The statement from Apple never explicitly said the company wouldn't throttle devices from last year, but the fact that it made hardware tweaks to avoid similar issues certainly suggested otherwise.
So, if you do update to iOS 12.1 and you and own a device from last year, you may notice slower performance going forward, though it may be less noticeable than when iPhone models released before 2017 were throttled.
If you're unhappy with Apple's throttling practices, you can always turn the feature off, though Apple doesn't recommend doing so. You can read more about iOS's Battery Health feature right here.