When a device is decommissioned you can decommission it via a button called " Decommission". This will removed the host from the host groups. It will also move the templates. It would go into a host group called Decommissioned" for like 60 days. If you search for that host you will see that it is decommissioned. There would be a button to bring it back into live status with the host groups/templates back in place.