That the PlayStation 5 would be able to work with PS4 games is nothing new. Sony itself had already announced that it would make at least the top 100 of the current console compatible with its future device, and some sources even say that players will have the freedom to test any PlayStation 4 disc in the novelty when it becomes available.
Ubisoft, however, arrives to rule out once and for all the possibilities of the PS5 being compatible with older models, such as the PlayStation 3, 2 and 1.
Okay, that was unlikely given the different architecture of these platforms, but the unofficial confirmation offered by the developer comes through a support page of the company, in which details the cross-play between PS4 and PS5 games. Multiplayer between these systems will be possible.
PS4 games may still make use of DualShock 4 on PS5, but the old controller will not work for native PS5 games, which will require a DualSense.
Information about non-compatibility with PS3, PS2 and PS1 games, in any case, comes through information from Ubisoft, not Sony. But of course, close partners as they are, the Japanese would not leave the developer uninformed to the point of publishing wrong information about the upcoming console.
It is also worth remembering that many popular and classic games from these old consoles have already gained ports or remakes for more current systems. So, if you really miss a classic from your childhood, it is quite possible that a new version of it will be available on PS4 itself or in the future on PS5.