[savoytruffle]

It supports iPads with an A8 series chip, while iOS 13 does not support such iPhones (which would be a 5S). It's presumably because the iPads of the same CPU generation usually have more RAM, more CPU cores, more GPU cores.

[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.

[saagarjha]

iPad Air 2 has an A8X, and iPhone 5s has an A7.

[dep_b]

It looks like the cutoff was 2GB of RAM.

[Miraste]

Those decisions are pretty arbitrary. Way back on iOS 5, the iPod Touch 4 got the update and the original iPad missed it, despite using the exact same processor and RAM.

[unicornfinder]

Keep in mind though that in the iPad it's having to push a lot more pixels.

[Miraste]

Not back then. This was before "Retina" iPads, but the iPod Touch was Retina. You had a 960x640 screen vs a 1024x768 one. More pixels, but not so many more that it should block an OS update.