You can bet your bottom dollar any iPhone or iPad with A9 (not A9X) will be able to run iOS 12 fluidly. iOS 13 will run but may or may not be fluid.
Unlikely these devices will be offered iOS 14, especially SE/6s, and in case they are offered iOS 14, you can bet your bottom dollar it will run like a POS. There is nearly no chance that not only is iOS 15 offered to these devices but also runs as fluidly as iOS 11 runs on these devices.
[doublepost=1529569303][/doublepost]It isn't only about iOS supporting your device, but also running it well.
To give an example, I am mentioning iPhones with the last iOS version they ran pretty well.
iPhone 6 - iOS 9 (maybe iOS 10 tops but unlikely) (excluding iOS 12, which I don't know, but iOS 10/11 was crap).
iPhone 5s - iOS 8
iPhone 5 - iOS 10, yes, it was faster than 5s on iOS 10 from videos at least.
iPhone 4s - iOS 7
iPhone SE/6s and above are fine with iOS 11/12+ for now.
I don't think any current iPhone or iPad will run anything beyond iOS 13/14 decently, at the very max. Sure, they'll be supported but don't expect them to run iOS 15 decently. Maybe not even iOS 14.
iPhone 8 Plus and X will run iOS 13 okay/fine I guess. iPhone 8 may or may not be as good thanks to 2GB RAM.