For anyone else wondering, to enable it just go to iOS Settings -> iCloud and you'll see "Advanced Data Protection." Toggle it to enabled to create a recovery key, which you'll then be prompted to input correctly after saving it somewhere safe, and then return to the iCloud Settings page, toggle it one more time and enter your recovery key again to confirm.
For anyone else wondering, to enable it just go to iOS Settings -> iCloud and you'll see "Advanced Data Protection." Toggle it to enabled to create a recovery key, which you'll then be prompted to input correctly after saving it somewhere safe, and then return to the iCloud Settings page, toggle it one more time and enter your recovery key again to confirm.