FSR1 is a simple frame upscaler, it’s used as one of the methods for resizing the image when you render something at s lower resolution and stretch it to full screen.

FSR 2 and 3 are temporal (time based), they need to be implemented directly by the game engine but if they do (like Deep Rock Galactic), they work on the deck.

FSR4 officially needs an RDNA 4 based gpu, which the deck doesn’t have, but the plugin/mod converts DLSS in to RDNA 3 compatible version of if.