The Use 2d Physics option will enable 2d physics which is not what I’m suggesting. I’m suggestion the
"New: Simple 2D Physics Constraints
The engine now supports easily constraining 3D physics objects into a 2D plane. This should help with 3D side scrollers."
…which was added in 4.4. have a look at the release notes…
It says…
'The setting can be applied per object, or project wide by changing the project physics settings."
So go to world setting under the physics section and the option should be there.