As a sibling commenter puts out, a uniform deletion "certificate" or other notice that could be used as verification of requests would be useful to ensure that

As a sibling commenter puts out, a uniform deletion "certificate" or other notice that could be used as verification of requests would be useful to ensure that audits would have a corresponding record to use to determine what records should not be present. Then, data broker keeps a list of such deletion notices. If any of those notices ends up having data still stored, that's a violation. If any customer presents such a certificate but isn't in the list of such notices on the data broker, that could be a violation. This way there's an effective, enforceable mechanism. Otherwise, a broker can just lose deletion requests entirely and no one would know.