Apple introduced ProMotion display tech on iPad Pro back in 2017, providing a variable update rate of up to 120Hz. There have been rumors that the same technology would make the way to the iPhone, but it has not been materialized so far. According to the gambling Ice Universe on Twitter that can be changed on 2020 iPhones.
The leak says Apple is mulling 120Hz panels on next year's iPhones. The company talks to Samsung and LG over screens. The technology itself will probably remain unchanged from the iPad Pro ̵
– Ice universe (@UniverseIce) July 21, 2019
Unlike iPad Pro, which uses an LED backlight Retina display, the current iPhones have a OLED panel, with Samsung production screens. So it's only sensible that Apple talks to Samsung and LG – two of the largest manufacturers of OLED monitors – across the screen that will end up next year's iPhones.
The 120Hz iPhone rumor has made the rounds for a few years, but with some Android phones with high refresh rate panels, 2020 can finally be the year we see the technology on iPhones.