Samsung claims the Galaxy S8 Microsoft Edition was just a myth
The Galaxy S8 Microsoft Edition should have been available for purchase at Microsoft's retail stores, and while they didn't include anything different in terms of hardware or software over the standard models, a Microsoft customization was applied to these phones when the devices were unboxed and connected to Wi-Fi.
Basically, the customization was supposed to raise awareness of the Microsoft's apps like Office, OneDrive, Cortana, and Outlook. Probably nothing more than a PR stunt, but something that certainly attracted some Microsoft fans.
Fast forward several months and Samsung says that there was never a Microsoft Edition of the Galaxy S8, or of any of its other flagships.