It looks like you get a warning when you try to launch the PS4 version of a cross-platform game on PS5, asking you if you wanna stick with the PS4 version or switch over the PS5 one.
This feature was found by Twitter user @Tidux so far. The warning gives you two options, which are “Play PS4 Version” and “Switch to PS5 Version
This feature could come in handy when a game doesn’t indicate which version of it you are in. For example, there were complaints about Call of Duty: Black Ops Cold War because it wouldn’t show which version of the game you are in when in-game.