I had 68GB of System Data and couldn’t get it to move much at all no matter what I did. I even manually deleted nearly every app to get rid of its associated data and even that didn’t get me below 60GB.

The only way I’ve found to wipe out nearly all of the system data is to factory reset the device and set it up as a new phone. Of course, you’ll lose EVERYTHING that’s not backed up to iCloud, Dropbox, etc so be careful. You will even lose device settings like haptics and sounds. After doing that and reinstalling all my apps, I had about 2GB of system data.

My theory as to why I had so much: I have not set up any of my last 4 or 5 iPhones up as a new device. I have been just allowing the old phone to load everything onto the new phone each time. I’m guessing after doing that several times, I ended up with a large pile of orphaned data that the phone had no idea what to do with. I could be way off on this; maybe an expert can set me straight.