After initially testing iOS 15, which found that many third-party apps could not exceed the standard 60Hz refresh rate of the iPhone 13 Pro's ProMotion screen, Apple has clarified the situation. The good news is that developers will soon be able to take full advantage of the 120Hz refresh rate display.
Apple says that developers will need to add a new Info.plist key for the feature to work, and documentation on this is coming soon. In addition, an upcoming software update will fix a bug that caused Core Animation-driven animations to fail to reach their maximum 120Hz refresh rate.
Standard animations and scrolling interactions are automatically enhanced with ProMotion without the need for developers to write custom code. This functionality includes power savings when the screen can go down to 10Hz, rather than just peaking at a smooth 120Hz.
For battery saving reasons, applications will need to drop 120Hz support if they are performing custom custom rendering, for example in games. The full list of steps developers need to take will be officially documented shortly.