Nobody can tell for sure, but my feeling is that Apple so far has tended to upgrade beyond the level where it's debatable whether it's not even worth upgrading even if possible. That is all ipads except the first have been upgraded to IOS 9, which was the big turning point and which sort of killed 500MB devices and made 1GB RAM devices painfully slow. By the way IOS 12 does not run worse than IOS 9 on those devices.... Also since ios 11 no 32 bit app works on ipads... My ipad mini 2 on ios 10 is generally slower than my ipad air 1 on ios 12, but is pretty fast with old 32 bit apps (they probably require less RAM/CPU).
So if IOS 13 and 14 will increase RAM usage significantly then ipad pro 9 might slow down. However Apple may simply disable some multitasking features and still upgrade like they did with split screen on 1GB devices. Still, even without that those devices were hit hardly...
My guess is that apple will support the pro 9.7 until at least IOS 14 and very likely IOS 15 too. The big question is whether it will be worth upgrading or not.