Like it or not, the A12Z is literally just an A12X with an extra GPU core. Just because Apple released it on a later date, doesn't make it an entirely new chip. And looking at how Apple provide support, I'd expect Apple to drop support for both A12X and A12Z at the same time despite the 2 year release gap.I already am planning to upgrade my 12.9 Pro to have a Cellular model again, but I'm holding for the M2 lest Apple screws me over twice a la A12Z not getting most of the upcoming iPadOS 16 features on a 2-year old chip.
Side Note:
The A12Z chip is only 2 years old. It came out in 2020. The A12X chip came out in 2018. They are not the exact same chip, no matter how many people claim otherwise.