I have an old iPod touch which cannot be upgraded past iOS 3.1.3 and the (currently) newest version of iTunes (18.104.22.168). iTunes does not allow me to create a backup of that iPod to my Mac (the buttons and settings in the Backup section are just grayed out). That's not terribly bad, but it got me interested: Is there a minimum iOS version or other restriction that prevents recent versions of iTunes from creating a backup of a device? Is this documented somewhere?
I think it's important to know because I may want to migrate my data to a newer device, regardless of how old the old device is.