I have seen people very confused about which games will run on their system, though. Most are still cross compatible with XB1 and Series X, but some are Series X only now and the boxes aren’t marked clearly enough for some people to tell the difference.
I just double checked, and it didn’t have any, it looks like. So it’s just the PS3 Slim that cut backwards compatibility.
As for why the PS4 wasn’t backwards compatible with PS3 games, the PS3’s architecture was hard to emulate and too expensive to include wholesale without ballooning the price beyond the target.