Ehhh let me put it in a different way - it's not impossible, of course. But it's about the fact that if someone is using a PS4 controller and they don't know about this, then they are going to have a bad time. And no dev wants their players to have a bad time, and neither does the platform holder. So they would rather just say no, you as a dev don't have to worry about this - every player will use a PS5 controller, done.

>>what about PS5-only games that happen to exist on other platforms that don't have such features? Can they be played with a PS4 controller?

Sure, Ratchet and Clank has a PC version now which can be played with any controller, including a PS4 or xbox controller. Obviously for this version it was remapped to make sense when using such controllers.

Again, it's not about actual literal impossibility - it's Sony's choice to say "look we want to promote features of the PS5, so code your gameplay features to make use of the adaptive triggers and we guarantee that every player will have a ps5 controller. You don't need to code your game to support older pads" - so devs don't. They obviously do on platforms like PC where the player might be using anything.