Compatibility Plugin
Developer Note
Since this version of Joomla has not been released yet, this page can change anytime.
Update Needed
No decision has made yet what's happening to this plugin in Joomla 6.0
As part of Joomla! 5.0 a plugin was introduced which enhance backward compatibility between Joomla 5 and 4. The plugin is implemented as "Behaviour" plugin type to guarantee that is loaded before any other plugin is loaded. (Just a heads-up, don't create a plugin as behaviour plugin because it's possible that this group get removed at some point)
For more detail check Compatibility Plugin Joomla! 5.0.