You can shred the adverse action letters (and files) anytime you want to (unless contrary to state law). Instead, you can retain the information electronically. Many examiners will request electronic access to documents rather than paper copies. As mentioned above, for ECOA, you must retain documents for 25 months, so the 2017 data less than 25 months still needs to be retained.
Integrity. With it, nothing else matters. Without it, nothing else matters.