You're probably barking up the wrong tree if you want to use ARKit with the older chips. You could look into another framework entirely though. There's augmented reality apps in the store that pre-date ARKit. But why develop looking in the rear-view mirror.? ARKit is pretty amazing, I'd highly recommend using it.
Thanks for response.
There are still plenty of iPads with A7/A8 although I would be interested to see some Apple analytics/volumetrics on what the current distribution v. chipsets is. After throughly reading the documentation, I did not notice that there is one feature which I was interested in using but unfortunately it is exclusive to the iPhoneX. In addition, a highly tuned gpu framework for A7/A8 will increase by ~2 orders of magnitude on A9, ~1.5 on A10.
I have been looking into other frameworks, Google's is too much of a hassle and I don't like the idea of wrapper around already slow java implementation. FB seems to be more on the cutting edge. I created a full 3d AR/VR model with 20 bones, 4 vertices per node, animatable in blender and had it playing via FB's AR Studio through the Messenger camera in under two hours.
The poor man's solution would just be to overlay scenekit/spritekit on a highly optimized metal video view.
Plus I'm too cheap to upgrade when my dev iPad works just fine.