The LDAP and database authentication backends have been usable together since GUAC-586, but this still causes trouble in the case that only LDAP users that also exist within the database should have access.
There are cases where large deployments of Guacamole involve a large LDAP tree that contains many users, only a subset of which should be granted access to Guacamole. Restructuring the LDAP tree to ensure that only certain users can log in to Guacamole is not always feasible. Rather than universally granting access so long as LDAP accepts the credentials, the database authentication should provide an option to deny access to authenticated users if they do not also have associated data in the database.
It has been verified that extensions can indeed reject an otherwise positive authentication result from a different extension.