At the moment, Valve and Microsoft have not commented on this discovery.
A few days ago in the Steam Beta repository service GitHub has added eight new files updates in which inquisitive users almost immediately found a new place xbox_pairwise_id in the sub network identification Steam. It is logical to assume that it can be used to link accounts to Steam and Xbox Live and the subsequent cross-platform play between PC users and Xbox One consoles.
At the moment, Valve and Microsoft have not commented on this discovery. It is worth noting that projects such as Rise of Nations: Extended Edition, and Killer Instinct already support cross play between platforms Steam and Xbox Live. In addition, cross-platform multiplayer has a place in Rocket League (between PC and PS4, and between PC, Xbox One and Nintendo Switch), Fortnite and other projects.
Microsoft recently announced plans to increase its presence in PC gaming. The unification of the platforms of the Valve would serve as a good guide in these intentions. In addition, Microsoft can start releasing more of their games on Steam, which will be appreciated by most players on PC. Finally, similar steps can add points Xbox One in its confrontation with the Play Station 4.