Neither of those. It's been a loooong time now that Ubi$oft stopped caring about their older games, stutted this game's dedicated servers[www.ubisoft.com] and this is just one case amongst many of those within their titles.

Those saves are located inside your WIndows OS (Operating System), usually at this location: "C:\Users\Insert User Namer here\Saved Games\Assassin's Creed Brotherhood\". Simply make a backup copy of that entire folder and store it away at your external Hard Drive or some USB Pen (with a small text file along telling you where you need to place it the next time you install the game and want to play it, because that "ACBrotherhood.ini" file also keeps your all game's definitions, if not mistaken).

Originally posted by Basan™:

Neither of those. It's been a loooong time now that Ubi$oft stopped caring about their older games, stutted this game's dedicated servers[www.ubisoft.com] and this is just one case amongst many of those within their titles.

Those saves are located inside your WIndows OS (Operating System), usually at this location: "C:\Users\Insert User Namer here\Saved Games\Assassin's Creed Brotherhood\". Simply make a backup copy of that entire folder and store it away at your external Hard Drive or some USB Pen (with a small text file along telling you where you need to place it the next time you install the game and want to play it, because that "ACBrotherhood.ini" file also keeps your all game's definitions, if not mistaken).

Strange because AC II seems to use the ubisoft cloud saves just fine. Now that I'm on this game I can't seamlessly go from my steam deck to PC and I'm pretty saddened by it.

That's because, if not mistaken, there's only a version being sold of Assassin's Creed II (AC II), contrary to Assassin's Creed: Brotherbhood (AC:B).
Here's a list of Ubi$oft'd different game versions saves[github.com], located inside their Ubi$oft Connect's folder installation directory, which is the following: "...\Ubisoft\Ubisoft Game Launcher\savegames\[Insert Rubbish Characters that are your Ubi' account in here]\... and there you must go to manually find your saves.

Although you're in luck, I'm currently replaying AC:B and recalled that back in those days Ubi$oft still weren't that advanced in trying to prevent users form manually finding their AC:B's saves and therefore you ought to look insidde this location: "C:\Users\[Insert User Name Here]\Saved Games\Assassin's Creed Brotherhood\SAVES\".