I wonder about those Ubisoft games, as Syndicate and a few other Assassins Creed games were said not to work with PS5.
"Were said to not work" where? Are you referring to the Ubisoft article "“We have pulled the Ubisoft Connect article and forum post re backwards compatibility for the time being as there may be inaccuracies involving the Ubisoft titles that will be playable on PS5.”"
You can use the user-made https://andshrew.github.io/supreme-enigma/ to search Playstation's search for whatever game you are looking for.
All five variants of Syndicate is listed as "bootable". When an option such as "not_compatible" is a choice then "bootable" means there is a level of compatibility.
None of the assassin creeds games are listed as "non_compatible" and even at least one of those games found on that now deleted article we even listed as 'compatible' not just bootable.
You are bound to love Earthbound.