Most recent phones have Always-On Displays, not weather someone uses it or not is varied and impossible to know. But given the technology is not new and been used for many years on OLED screens means it’s a very low risk of burn-in.Consider how people use phones. Check an email here, make a call there. Play a couple mobile games for a bit. Most phones are not left with the display on for 8 or more hours a day. It's the length of 'on' time that can cause the burn in, lesser so the frequency where the screen can rest.
To my knowledge burn-in increases when the screen is too bright with static images on for a prolonged amount of time. Apple is quite capable of incorporating dimming technology for some of these static element such as the menu bar, dock and desktop items. Think Apple wouldn’t have a solution within the OS vice looking forward to warranty repairs.