Source: ValveCode found in a Steam Beta update suggests that Valve and Microsoft are planning to improve cross-play between PCs and Xbox One consoles. The code refers to Xbox-related networking features, which could mean a future version of Steam will make it easier for devs to enable cross-play.
Valve hasn't officially acknowledged the code in question, but it wouldn't be surprising to learn that it's collaborating with Microsoft to help bridge the gap between PC and Xbox players. Microsoft has spent the last few years unifying the platforms by introducing Xbox Play Anywhere, making an Xbox Windows 10 app and revealing initiatives like Project xCloud that will allow people to stream Xbox titles regardless of what device they own.
It would make sense for the unification of these platforms to continue with additional cross-play support. But even though it sells games via the Microsoft Store on Windows 10, it's doubtful Microsoft could accomplish much on its own. Too many people use alternative marketplaces, from mainstays like Steam and GOG to newcomers like Discord and Epic Games, for Microsoft to make a meaningful change without some assistance.
That's where Valve comes in. Competitive game marketplaces have been targeting Steam for years, but most have simply managed to live alongside it, not replace it. Steam also offers features to make it easier for developers to include controller support in their games, offer multiplayer game modes or accomplish many other things besides. Valve's offering is as close as PC gaming comes to having a unified platform.
The game industry is trending towards cross-play. Microsoft and Nintendo have more fully embraced the concept when compared to Sony, which objected to letting PlayStation 4 owners play with people who own other consoles, at least until the backlash from its stance reached a fever pitch. But even Sony had previously committed to letting PS4 owners play with PC and mobile gamers. (No word yet on the Switch doing the same.)
We've reached out to Valve to see if the company might offer more information about the Steam Beta update and will update this story if it responds. And it's worth remembering this is an update to the beta version of Steam--this could be something the company tests but never actually releases to the public. With the way things are trending, however, enabling native cross-play between Steam and Xbox seems more than likely.