So old and nobody touched it for years. It is evolving interface and not used by Solaris internals from time of "patch from hell". It was obsolete in Solaris 9, replaced by libldap.so.5 as public LDAP API.
It lacks all necessary things around modern LDAP (e.g. real security layer) so it is not very relevant today. The most of apps build on Solaris 8 with -lldap can use libldap.so.5 safely, if they even exists.