The DEK field is defined here:

/*!
 * Encryption data used by the WT_PAGE_LOG_HANDLE interfaces.
 */
struct __wt_page_log_encryption {
       char dek[32]; /* an encrypted version of the key used to encrypt data */
};

The security team used the dek information to verify a delta is correctly matching the same base page. This is now deprecated and not needed anymore because the security team uses backlink lsn to verify this behaviour. As such we can remove this struct from WT code.