[simonh]

It’s almost certainly memory. All the supported devices have at least 2GB RAM.

The iPhone 6 had the A8 with 1GB RAM and is not supported by iOS 13. The iPad Air 2 has the A8X with 2GB RAM and is supported by iPadOS 13.

In the past the break on unsupported older devices has been down to hardware. Usually memory, but also on the 64bit transition. I’m not aware of a single break in support that wasn’t determined by hardware requirements.

[close04]

The A8X also has a substantially faster CPU than the A8 (3 vs. 2 cores and 1.5GHz vs. 1.1GHz) and a much faster GPU (2-3 times faster). This would make a visible difference in day to day usage even without the RAM limitation. This is probably also what made the difference when implementing the multitasking features.