The new ASUS ROG Phone II gaming smartphone has a super-fast 120Hz display but it looks like the 2020 iPhone will have a "switchable" 60Hz/120Hz display. Apple will be sourcing 120Hz panels from either (or both) Samsung and LG, with discussions happening now.
We should expect Apple's new iPhone to possibly roll out with 5G connectivity, with fresh news popping off this morning that Apple was in "advanced" discussions to buy Intel's modem division. The move to a 120Hz panel isn't completely new for Apple as both of the 2017 and 2018 10.5-inch and 12.9-inch iPad Pro models feature a 120Hz display.
Apple calls the 120Hz screen technology ProMotion Display, which is a switchable refresh rate display that drops to just 24Hz with a static image on screen, and 48Hz when videos are playing. This is the big saver on battery life as it means the screen isn't constantly refreshing at 120Hz which would be a drain on battery life.