Windows 10 is constantly under development - every new update brings new features or removes some. The latest Windows 10 Fall Creators update removes the Windows Media Player from the system. More specifically, the update KB4046355 ("FeatureOnDemandMediaPlayer") deletes the installation of the Windows Media Player on a Windows 1709 installation. The Windows Media Player can be installed manually via the "Manage optional features" menu item in the "Apps & Features" menu.
Microsoft has not yet formally commented on whether the step means that the Media Player will be missing in the final release version. Many users still use the Windows Media Player, although it replaces the Films&TV Ap in Windows and much better media players like the free VLC Multimedia Player exist.