> How is it at all possible to certify that data has been deleted and no copies were taken..?
By, e.g., signing a paper which says that.
You probably meant how can you know that, which is a different issue than how can you certify it, and which amounts to a question of degree of internal accountability and control of data access.
Certification is about assuming responsibility for the truth of the thing certified, which doesn't actually require knowing it (though obviously knowing it to a reasonable degree of certainty makes it more comfortable to certify it.)
By, e.g., signing a paper which says that.
You probably meant how can you know that, which is a different issue than how can you certify it, and which amounts to a question of degree of internal accountability and control of data access.
Certification is about assuming responsibility for the truth of the thing certified, which doesn't actually require knowing it (though obviously knowing it to a reasonable degree of certainty makes it more comfortable to certify it.)