how to delete device no longer linked to account

If the iPad remains in your possession, you can fully erase the device by following this process:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

This procedure will also remove the device from your AppleID account.

If you do not have the iPad in your possession and you have manually removed the device from the listed devices in your AppleID account - you will have lost all opportunity to remotely manage the iPad.

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