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]