if you leave that new feature setting empty (which should be the default value if you upgraded from 2.2.3), then it will use the previously hardcoded remote username format. That is: DOMAIN\username.
Unless you modified the LDAP auth plugin code to use any other format in your 2.2.3 version, it should work as before (as internally it's using the same format). If you modified the code to use other formar, then simply specify that format in the new feature setting. Have a look at the help text next to the setting, to see how to specify your format.