If you keep the screen brightness all the way up, and have the same thing - like one particular home page - on the screen for hours a day, almost any screen will experience burn-in. But unless you're out in the sun all day, you rarely need more than 20% or 30% brightness. And that will either eliminate burn-in, or cause it so far in the future that you wouldn't want an old phone like an S9 any more. It doesn't have 5G or 6G, it doesn't do peer-to-peer calls, etc. - things we'll probably see in coming years. (Look back at the "flagship" phones of 2004.)