I just connected my Mac OS X 10.6.8 Mac to iOS 11.4.1 running iPad Air 2. Image Capture was able to see the photos and movies to move over to the Mac OS X 10.6.8 system. As suggested in my prior post, this means everything except ripped music will still be able to sync to iCloud, and your older Mac. It just won't be with Mac OS X iTunes, if you have exceeded the iOS the iTunes on your Mac supports.
And you can add external storage via Lightning or WiFi that has your ripped music which would just need to be organized by playlist, instead of by album.
This is a workaround, but it saves you from having to buy a newer Mac, another iDevice with an older iOS preinstalled, worry about the lack of the iOS being available with the digital signature online, or having to buy excessive storage on the cloud to backup your photos and movies when the iOS has gotten too new.