Responding to a tweet about cross-gen support, Insomniac bluntly explained that Rift Apart is indeed "a PS5 exclusive." It will not be available on PS4, making it a true-blue next-gen game.
In another timeline, this kind of clarification would be fairly inconsequential, but next-gen exclusivity has become a central and confusing topic for upcoming PS5 games ever since Horizon Forbidden West, Marvel's Spider-Man: Miles Morales, and Sackboy: A Big Adventure were confirmed for PS4. Sony stressed a clear generational leap for months and then abruptly revealed cross-gen support for many leading PS5 games, so the PS5 exclusivity of games like Demon's Souls (which we now know is also exclusive to PS5, and will not available on PC despite some marketing materials saying otherwise) and Ratchet & Clank was called into question.
It's a bit aggravating that this kind of information is just now trickling in from innocuous tweets rather than, perhaps, Sony's many PS5 showcases, but it's good to finally know for sure that Rift Apart is actually a PS5 exclusive. While it lacks a firm release date and won't be available at launch, Insomniac and Sony previously confirmed that it will be released within the console's launch window, so it should be out within four months and change. Ratchet has been a tentpole franchise for generations of PlayStation consoles, and it looks like PS5 will be no different.
Ratchet & Clank: Rift Apart has a 60 FPS mode on PS5, which also implies a lower FPS mode that prioritizes graphics.