A recent Ubisoft support page seems to indicate PlayStation 5 may only be backwards compatibility with PlayStation 4.
As the support page states: “PlayStation 4 players will be able to join multiplayer games with PlayStation 5 players… Backwards compatibility will be available for supported PlayStation 4 titles, but will not be possible for PlayStation 3, PlayStation 2, or PlayStation games.”
This falls in line with a hardware showcase that happened in March. In it, there was a screen which showed backwards compatibility only announced for PS4 Pro and PS4 games.
Xbox, on the other hand, has been fairly clear about their backwards compatibility. In a post by the company, they have claimed that the Xbox Series X will be compatible with all previous generations of Xbox games.
However, it should be noted that Sony itself has not made an official statement on whether or not the PlayStation 5 will be backwards compatible with PlayStation 1, 2 or 3 games. This Ubisoft support page could be referring specifically to Ubisoft games and the company’s approach to backwards compatibility.
[Update 9/1] The Ubisoft support page has removed mention of backwards compatibility.