I've been pretty adamant in previous threads saying that nothing exists on a system level for PS5 to facilitate PS4-to-PS5 save sharing but I'm happy to eat my words if that isn't the case.
However, I'm still suspicious because so many developers are implementing it for Xbox and not PlayStation that at the very least it's nontrivial to add.
It's a difference in how Sony's databases work. Microsoft is treating cross-gen and even cross-platform games as the exact same SKU, whereas Sony treats the same game on different platforms as seperate SKUs. It's why Microsoft has smart delivery whereas Sony doesn't have anything of the sort.
Sony has been up front since PS3-PS4 about how cross save works it's always been up to the Devs. This is what people have been saying with games now doing the same thing.
It's up to the developers on Xbox either so... why do Yakuza 7, Dirt 5 and Maneater support cross-save on there whilst they don't on PS5? As I said, framing something as a "developer decision," literally just means that it's not enforced-via-contract by the platform holder. So, in the end, means literally nothing when the important issue is what that 'developer decision' actually requires to implement.