UPDATE: Obviously this isn’t a fix for everyone, but I was able to find some information for the contingency where a hub fails (or in this case, the hub requires unpair functionality). If you have ANOTHER hub (which in my case I had an old Nexia hub), you can perform the following to UNPAIR devices:
1 - Activate the secondary hub
2 - Ensure that at least one device is paired to the secondary hub
3 - Initiate an unpair action on that hub using the sensor locked to the SmartThings hub. For the door sensor, this involved puling the battery for 1o seconds, initiating the unpair action on the secondary hub, then putting the battery in.
Apparently - this action force resets the sensor to factory conditions, and it can then be paired with the SmartThings hub again.
NOTE - There is still an issue with the sensor, where it is stuck in the “Open” position. I have confirmed that my sensors are good using the old hub and they do work. Seems like a configuration needed for the ST hub.
Hope this helps…