--pgp6 flag. This is not nearly as involved as --pgp2. In short, it

turns off force_mdc, turns on no_comment, escape_from, and force_v3_sigs,
and sets compression to 1.  It also restricts the user to IDEA (if
present), 3DES, CAST5, MD5, SHA1, and RIPEMD160. See the comments above
algo_available() for lots of discussion on why you would want to do this.

g10/options.h

@@ -79,6 +79,7 @@ struct {
     int compress_sigs;
     int always_trust;
     int pgp2;
+    int pgp6;
     int rfc1991;
     int rfc2440;
     int pgp2_workarounds;