/usr/lib/ldap/deref.so is in slapd 2.4.31-1+nmu2ubuntu8.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ldap/deref-2.4.so.2.8.3
/usr/lib/ldap/deref-2.4.so.2.8.3