PS2 base models were BC to PS1 PS3 base models were BC to PS2 (and PS1) even before they added digital-purchased emulated copies. PS Vita has a built-in PSP that can play digital versions of those games as well as PS2 ones. PS4 had a very limited selection of PS2 games released digitally for it.

PS5 is BC with PS4.

They could do a lot better, though. Their digital offerings are extremely limited and have yet to tackle the issue of emulating the cell processor at reasonable speeds.