No data is lost and only the RAID1 device gets deleted under the following scenarios:
When the RAID1 device has only one segment and if the device is consumed by a pool, deleting the RAID1 device deletes only the device. The segment is directly attached to the pool.
When the RAID1 device has only one segment and if the device is an SBD mirror, deleting the RAID1 device deletes only the mirror. The mirror's segment becomes the SBD partition.