If you purchase Call of Duty: Infinite Warfare on Windows 10 through the Windows Store, you will not be able to play with users on Steam, and vice versa — same goes for the remastered Modern Warfare.
The unfortunate news was revealed in a recent Activision FAQ that simply states:
“You can only play these titles with other users of Windows 10 on Windows Store.”
There will also be no cross-platform support between Windows 10 and Xbox One players. When questioned, Microsoft sent the following statement to Windows Central:
“We support cross-play between devices and platforms for partners who want to enable it.”
So it would seem Activision made the call to separate the PC audiences.
Call of Duty: Infinite Warfare released today on PS4, Xbox One and PC.