Customs and Border Protection has nothing to do with enforcing the test mandate (for all air travelers) or vaccination mandate (for non-citizens/LPRs). It is a CDC order that applies before a flight departs a foreign location to fly to the United States. The airlines and private aircraft operators must comply with the order and maintain records, including the attestation forms (whether digital or paper). CDC could audit any flight at any time, so the paperwork must be on file.