This preserves the encryption algorithm that had been used in a now-expired control tunnel, per its description, the historical companion to the live cisgIpsSgTunEncryptAlgo. It's the forensic record an admin needs when investigating a since-closed connection's actual cryptographic strength, for example when a security audit needs to confirm what cipher protected a session that's no longer active to check directly. A history entry showing 'des' for a tunnel to a partner an admin believed only ever used AES would be a specific, actionable audit finding about that partner relationship's actual historical crypto usage.