Erasing an old device
How do I erase an ipad I no longer have in my possession
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That would largely depend upon why the iPad is not in your possession. If the iPad is lost or stolen - and the Activation Lock (i.e. Find My iPad was enabled prior to its loss), then you may be able to trigger a remote erase.
Here is Apple’s advice for lost or stolen devices:
If your iPhone, iPad or iPod touch is lost or stolen – Apple Support
Use the Find My app to locate a lost or stolen device – Apple Support
If you have sold the device to a new owner, if properly prepared prior to the device being handed to the new owner, then you have absolutely no access or control.