A few rendering issues from the move to Metal but no KPs or major incompatibilities.
Sooner or later, things stop running. On the iOS side, I was surprised to learn you can’t run Netflix on an iOS 9 device.
I think the days of hanging on to old system versions are over.
https://www.lifewire.com/download-apps-to-ipad-using-itunes-...
Once it shows up as a previous purchase in your account, you can download it from your iPad. You don’t have to sync with iTunes.
"Extended support ends in September 2019. iTunes, in August 2020" [1]
And while High Sierra (10.13) had its quirks [for which I could understand your response, plus all non Retina only work with 10.13 as latest, officially), Mojave (10.14) has been smooth. If not only for the dark mode (finally!).