If the Air gets an A14, it could be supported longer than the A12X/Z. If the Air only gets A13, it might be supported the same length as the A12X/Z. It’s sort of an unknown.
We actually haven’t seen Apple stop support for any iPad Pro thus far. The very first iPad Pro from 2015 is still supported on the latest iPadOS. That device has A9X with 4GB RAM, so it could see support longer than all the other A9 devices, since they only have 2GB RAM. We don’t know yet.
RAM has been the deciding factor before. The iPhone 6/6 Plus lost support, but the Mini 4 and Air 2 haven’t yet. They all have a version of the A8 chip, but the iPads have 2GB RAM, while the phones only had 1GB RAM.
The next devices to definitely lose support will be the Air 2 and Mini 4 probably next year. I don’t know if they’ll retire the A9 devices too. I guess it depends on how demanding iOS/iPadOS 15 turns out to be. But I’d say even an A12 has 3-5 years left, so the latest iPad Pros and the supposed upcoming iPad Air 4 should all be fine for quite some time.
Just my two cents, for what it’s worth.