FYI. We were using an iPhone 6 and older ipad for ARKit dev and after upgrading SDK, it killed off support for those devices.
They had been working though the tracking wasn’t that great.
The new list of supported devices seems more enforced.
iPhone 6S and 6S Plus
iPhone 7 and 7 Plus
iPhone SE
iPad Pro (9.7, 10.5 or 12.9)
iPad (2017)
I can’t find any info on Apple’s docs anymore about the 3DOF fallback for IOS devices without A9 processors. I wonder if they scrapped that idea and are now strictly enforcing A9 and up only.
Just as an experiment though, have you tried explicitly creating an AROrientationTrackingConfiguration object instead of the default ARWorldTrackingConfiguration?
Details here - AROrientationTrackingConfiguration | Apple Developer Documentation
I was doing this through both Unity and Unreal and they just now stay with a blank screen. I confirmed it does work with 6s and iPad Pro.