Ubisoft Support Page: PS5 doesn't support PS1, PS2 or PS3 BC
... Crap.
https://support.ubisoft.com/en-AU/faqs/000055916PlayStation 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.
... Crap.