It appears that Microsoft will soon be including some sort of Steam tab in a future version of its Xbox App, as shown in a mock-up of a new bit of UI.
This leak comes via Eurogamer, who spotted in a blog post from Xbox an image in the header which shows the Xbox App open on a TV screen. If you closely at the image, you'll be able to spot a Steam tab next to Game Pass, Installed, Owned, and other tabs marking games in a user's library.
According to The Verge as well, Microsoft is apparently working on an update that will show users all the games they have installed on their PC, both from Epic Games and Steam as well as the Xbox Store.
We also heard that the next Xbox console will appeal to PC gamers as well, so perhaps this is the first step in the process of merging an Xbox and PC gamer more permanently.