http://www.pcgamer.com/windows-mixed-reality-headsets-may-not-support-steam-content-initially/

Microsoft recently jacked up the excitement level over upcoming mixed reality headsets for Windows 10 by announcing they would run Steam content. That helped allay our concerns (somewhat) that mixed reality experiences on Windows PCs would end up being gimmicky. Unfortunately, it appears that Steam support will not ready in time for launch.

Mixed reality headsets are set to debut when the Windows 10 Fall Creators Update rolls out on October 17.  We assumed SteamVR would be ready to go at the same time, but as reported by Computer Base, Microsoft's Greg Sullivan stated at IFA that it is going to take some additional time.