This preserves the authentication method used in the negotiations of a now-expired control tunnel, per its description, the historical companion to the live cisgIpsSgTunAuthMethod. It confirms after the fact whether a since-closed connection used pre-shared keys or certificate-based authentication, relevant when auditing whether historical connections complied with an authentication-method policy. A history entry showing pre-shared key authentication for a connection that occurred after a mandated switch to certificates would flag a policy violation that happened during that connection's lifetime.