It's arguably the first First Person Shooter game, recreated as per the title as a shader.

The enemies are Nazis, since few people would object to a game about an American soldier escaping a Nazi Castle and leaving a pile of Nazis in his wake.

Or at least, that was the case in 1992.

A very old FPS game, with the story arc being "break free from a WW2 Nazi prison" https://en.wikipedia.org/wiki/Wolfenstein_3D

If you google "wolfenstein", 100% of the first page results are related to the game series.

Wolfenstein, the game whose textures are being generated by this, was set in World War II Germany.

Just the Wolfenstein theme. See: https://en.wikipedia.org/wiki/Wolfenstein_3D

Wolfenstein is id's/Bethesda's "Super-Advanced-Secret-Nazi-Scientists"-Franchise. Hence Nazis.