There are "screen burn-in" apps on Play. (They basically turn on all pixels, one color at a time, to give the screen "even burn-in".) They do help a bit, but once the screen is burned in, it's burned in. This has been a problem since the days of CRT monitors, and continues today. (It's due to things at the atomic level, and there's nothing that can be done about it - except to keep the screen at the lowest brightness that you're not uncomfortable with. Even with that, all screens eventually suffer from burn-in, but most people don't keep their phones long enough for it to matter. [Except in cases like the earlier Pixel 2XLs, which suffered from burn-in in a few months at low brightness levels.])