Reliable iOS leaker @blue_kanikama has released a screenshot of internal iOS 14 code which shows a reference to 120Hz displays.
The image, which you can see in the original tweet below, seems to show an internal diagnostics program for testing. One of the diagnostics tests seems to be a test to scan and see if the device is utilising a 120Hz display.
This could could be in relation to the 120Hz Touch Refresh Rate that the iPhone XR, XS, XS Max, 11, 11 Pro, and 11 Pro Max use, even though their screen refresh rate is 60Hz. On the other hand, this could mean that Apple are working on an iPhone with a 120Hz display.
Many leaks and rumours over the last year have suggested that Apple would be bringing a 120Hz ProMotion display to the iPhone 12 series this year, although there have also been many conflicting reports saying that 120Hz ProMotion displays wouldn’t be arriving until the iPhone 13 in 2021.
iOS 14, alongside iPadOS 14, watchOS 7, tvOS 14, and macOS 10.16 are expected to be announced at WWDC20 on Monday. Recent rumours have suggested that iOS will be renamed to iPhoneOS and macOS X will be renamed macOS 11, although nothing is official until Apple announce it on Monday.
Credit to @blue_kanikama for this information!