Will ARWorldTrackingSessionConfiguration work on devices that don’t have an A9 processor. Looks like devices below the iPhone 6s do not have A9 chips. Don’t have that type device handy to try out a project. Hoping someone else has tried this. 
It’s not a problem of ARWorldTrackingSessionConfiguration: ARKit doesn’t work on devices below iPhone 6s. So, unfortunately, there’s no solution to your problem… unless you buy a newer model :]