Safely destroying Apple Software RAID 1
In general, Disk Utility forces you to erase a drive to turn on or off the RAID wrapper it places on the actual data.

If you set up a mirrored set (RAID 1) you still have access to the data and could just ignore the RAID wrapper since it allows you to add more drives as mirrors in the future, but doesn't force you to do so and you can keep using it as a single drive perpetually. I might turn off AutoMirror on that drive to ensure the OS never decides another drive is the master and proceeds to delete your data (an edge case, but I'd hate to experience this no matter how remote the chance).

The method Apple recommends to undo RAID is to back up the data elsewhere and delete the RAID wrapper and then copy the data back. My guess is if you erased the RAID wrapper and didn't otherwise do anything to the drive file recovery software could patch back together most if not all the files but I certainly wouldn't risk it for any data I wasn't prepared to walk away from.