In Toad version 13.0, after upgrading to an 18c client, the dropdown list on the LDAP tab of the connection window will be empty.
The reason is Toad is looking for the 12c ldap client dll in the 18c $ORACLE_HOME/bin directory.
Connections will still work; but if you want the list and search functionality you can restore it simply by copying the oraldapclnt18.dll to oraldapclnt12.dll in the same directory.
Update: Toad v13.1 recognizes the 18c client dlls correctly, thus obviating the need for the workaround.