Class map in J3: [https://github.com/joomla/joomla-cms/blob/3.10.12/libraries/classmap.php] Class map in J5: [https://github.com/joomla/joomla-cms/blob/5.0-dev/plugins/behaviour/compat/src/classmap/classmap.php] How to handle Classes that have no new counterparts like: - Jerror JError::raiseError() should be replaced with native exceptions. JError::raiseWarning() and JError::raiseNotice should be replaced with calls to application's enqueueMessage() method to display warnings/notices. More info on backwars compatible issues: [https://docs.joomla.org/Potential_backward_compatibility_issues_in_Joomla_4]