For the comments that mention that the device was removed (released) from the ABM/ASM (DEP) account, but the device is popping up the notification, this is normal/expect at this time. When the device checks in to Apple's activation servers, it downloads its activation record and sav...
If the device was decommissioned but not released from ABM, they should be able to release the product or you. Please note, you will still need to erase the device to remove the MDM enrollment unless they agree to issue a remove management command to drop the device from the MDM. They ...