You should be cautious when you delete encryption keys. When you delete an encryption key, you cannot restore the backup sets that you encrypted with that key unless you create a new key that uses the same encryption key and pass phrase as the original key.
See Deleting an encryption key.
You can delete encryption keys in the following situations:
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The encrypted data on the tape has expired or if the tape is retired.
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The encryption key is not being used in a job or a template. If the key is being used, you must select a new key for the job or template.
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The encryption key is not being used in a selection list for restore jobs and for verify duplicate backup set jobs. If you delete a key that is being used in one of the listed job types, the selection list can no longer be used.
If you delete an encryption key that is being used in a scheduled restore job, you cannot replace the key. Therefore, any scheduled restore job in which you delete an encryption key fails.