I imagine this is what steam was referencing during "Shmup" fest
Data from the image if you care to read
Legend:
Camera purist - a shmup must have a limited camera--either standing still, or automatically scrolling vertically or horizontally.
Camera neutral - a shmup can allow for greater camera and player control, allowing them to move freely in any direction on a 2d plane.
Camera rebel - a shmup can use any kind of camera and movement, 3D or otherwise.
Shooting purist - a shmup is a game where can only shoot in one direction (may have a secondary fire/"option" that goes in different directions.
Shooting neutral - a shmup is a game where your primary method of attack is shooting in any direction on a 2d plane.
Legend end
Data:
Shooting purist, Camera purist - Zeroranger is a shmup.
Shooting purist, Camera neutral - Fantasy Zone is a shmup.
Shooting purist, Camera rebel - Star Fox is a shmup.
Shooting neutral, Camera purist - Guardian Force is a shmup.
Shooting neutral, Camera neutral - Nuclear Throne is a shmup.
Shooting neutral, Camera rebel - Doom is a shmup.
Shooting rebel, Camera purist - any auto-scrolling level in Super Mario Bros. 3 is a shmup.
Shooting rebel, Camera neutral - Cory in the House is a shmup.
Shooting rebel, Camera rebel - Donky Kong 64 is a shmup.