In brief: Ubisoft and Steam parted ways about two years ago when the former decided to sell its digital PC games exclusively from Uplay and Epic Games Store. However, newly found evidence shows we may soon see Ubisoft games once again available on the popular digital games store.
As per the evidence found by SteamDB's creator, Pavel Djundik, it looks like Valve updated the Steam client and added the Ubisoft Connect PC client to the default package. If you have Steam installed and updated, you should already have the client included in your installation.
Valve and Ubisoft haven't officially commented, but if Djundik claims are valid, at least one of the two should announce it soon. The reason for Ubisoft's return is still unclear, but there's a good chance that it's linked to the Steam Deck's launch, scheduled for February 2022.
Ubisoft Connect PC Client was added to Steam's default package (all accounts have access to this by default), are new Ubisoft games coming to Steam?https://t.co/K6TRlJWPa3— Pavel Djundik (@thexpaw) November 29, 2021
Soon after the portable console's announcement, Ubisoft's CEO Yves Guillemot stated if Valve's portable console became big enough, Ubisoft would sell games via Steam once again. The truth is the console isn't even out yet, but the sheer amount of pre-orders Valve has received until now might be enough to justify Ubisoft's comeback.
Ubisoft does have some content available on Steam, including Assassin's Creed Odyssey, Trials Rising, Far Cry New Dawn and The Crew 2, and others. The last game added was Space Junkies on March 26th, 2019. Since then, Ubisoft launched many other titles, such as Assassin's Creed Valhalla, Far Cry 6, Tom Clancy's The Division 2, and others.
Moreover, the publisher is also working on a few Tom Clancy games, Avatar: Frontiers of Pandora, Prince of Persia: The Sands of Time Remake, Beyond Good and Evil 2, and more. These games are just some that might be coming to Valve's store if the French publisher decides to return to Steam.
Another possible reason for implementing the Ubisoft Connect client on Steam is to update already available games, but that is far less exciting.