This is something I learned the hard way using iMazing to handle my iOS updates.
I use iMazing to update my iOS devices because it forces me to do backups of the device prior to performing the update. Unfortunately, I was getting odd errors in iMazing when attempting to update to iOS v12.1. Well, it was at this point I learned the following from iMazing support:
“iMazing itself is fully compatible with MacOS 10.9 and earlier. However, the newest iOS 12.1 requires the newest Apple drivers, which can only be delivered by the newest iTunes version (12.9). Unfortunately, Apple makes it impossible to install iTunes 12.9 on anything but Mojave and there’s not much we can do about it. There’s no issue with updating to iOS lower than 12.1 on MacOS 10.13 and older, though.”