HEVC, also known as H.265, is a format in which some video files are encoded.
A lot of people, including me, reported that the native decoder needed to watch these kinds of video files is missing from the system after the Windows upgrade.

Applications such as VLC will still be able to play HEVC formats.

For those interested in encoding, instead, the native HEVC encoder is NOT missing from the system.

There's no known safe solution at the moment and since the issue broke some games and applications, Microsoft is currently working to fix the issue as soon as possible.