I wanted to say you can’t use last iCloud features in your new devices when you still own an old one which doesn’t support itI have actually never had problems with apps that have deep iCloud integration, like photos or notes. I'm currently using Catalina as my main system, everything works as it should, no problems at all. I also have an iPhone 5 with iOS 6.1.3 (no downgrade, no jailbreak), almost all iCloud services are working there as well, like shared albums, Game Center or synced contacts across all devices (minus App Store and iTunes Store, which was sadly shut down a few months ago for iOS 6 and below).
I mean of course, Apple is trying to push users to update and refusing to let them downgrade their iPhones and iPads, but I really can't complain about how long they keep their services fully working on old OSs. Also, you can download older versions of all Apple software that requires newer versions, if you have previously downloaded them, which is enough for most normal users.
For example, you can’t use rich notes or iCloud shared albums if you have a Mac on catalina