frustum projection is mostly the same as perspective projection but you have the ability to alter the frustum offset manualy.
Create a camera and look frome the side to see the frustum gizmo.
Then change the frustum offset y. You see the camera projection gets sheared.
This effect was used in old 2.5D shooter like Duke Nukem
What to do with this? I dont know! Maybe someone can shed some light on this!