Curious to know what does not work anymore? I guess youtube for one. I am on iPadOS 13 on my 9.7 (but I hardly use it anymore since it got screen issues), on 10 on my mini 2 (but I only use that for 1-2 apps) and on 15 on my 11" M1 (because that's the only way to be able to run Windows 11 virtualized). iPadOS 15 seems to be the minimum for a lot of apps, go to 14 and a lot more stuff is not compatible... but with 15 virtually everything is compatible.
YouTube doesn’t work anymore. With an overlay, it asks to update as soon as you open the app.
I had some issues with Netflix, too, but I tried on another iPad on iOS 12 and it works. It would be surprising, as Netflix recently worked on iOS 10 for me. Maybe I have to try again.
Safari is very incompatible now, with many websites staying blank.
The Reddit app stays blank now, too. All of these apps barring Safari worked until not too long ago. If I start looking for more, I’ll probably find more stuff.
iOS compatibility has improved, but six versions appear to be a little too much. It’s not even my secondary iPad now so it doesn’t bother me too much, but as a fan of the 9.7-inch iPad Pro it is a bit of a shame.
iOS 12 isn’t as good as iOS 9, but it’s decent.
I have iOS 10 on an iPhone 6s, and I reckon it’s not too usable. If you use your Mini 2 for a couple of apps I reckon you’ll be fine, though.
My iPad Air 5 on iPadOS 15, like you said, seems totally fine for now.
We are both lucky enough to have several devices on multiple iOS versions, so we can play this little game of switching devices for compatibility and assigning different usage patterns based on that. Sadly, many cannot do that.
I’ve had to update a family member’s iPhone 11 on iOS 14 to iOS 18 for compatibility reasons. Surprisingly, battery life is completely fine. There’s a lot of lag on some apps, though. The music app is a disaster, for example.
But it doesn’t matter too much, because for the iPhone the main aspect is battery life (which collapsed with iPhones like the 6s, I would know, I have one on iOS 13 and iOS 15 is presumably even worse). Relatively small performance issues aren’t an impediment. Disastrous battery life is, and the 11 has apparently been spared for now. I expected it to be a disaster, and it matched my testing on iOS 14 after I updated.