Yes, it is still needed as there must be user/group information stored together with task data to be able to query it later on.
Main difference is that you don't need to prepopulate db any more when using callback, task service will make sure that users/groups that are "approved" by callback will be inserted into db.
Thanks. Just to confirm again, when we connect to LDAP / add new users to LDAP while the task service is running, that will get added to the Organisational Entity table automatically. Am I correct about it?
Yes, as soon as task service will use these new users.