Microsoft previously announced that the upcoming mixed reality headsets for Windows 10 will be able to support SteamVR content and this blew up as it would allow more people to access VR content easily. However, it would seem that Steam support will not be seen in time for the launch.
These MR headsets will be arriving in time for the Windows 10 Fall Creators Update which is set for 17 October but Microsoft’s Greg Sullivan mention during IFA that it will take some additional time before we see the support. He did not give a time frame but he did say that they have only just begun to bring SteamVR support to the headsets and there could be a chance that we will only see the support next year.