I mean the standard physics in unreal engine which is the nvidia physX engine under the hood. There is an option from 4.4…
"New: Simple 2D Physics Constraints
The engine now supports easily constraining 3D physics objects into a 2D plane. This can help when making 2D games like side-scrollers."
Looking here…
…I see that yes you were right in having use 3d physics. I got confused with the physics 2d ‘constrain to plane’ .The forum docs say…
“Use 3D Physics: Collision geometry will be generated for use with PhysX. The 2D collision geometry defined in the sprite will be extruded to “Collision Thickness” units deep in the perpendicular axis to make 3D collision geometry.”
So I would have Use 3d physics ON and then in the physics section of the actor, have “Locked Axis” set to be the plane that your 3d world is aligned against. I would like to test this myself.