Compatibility Plugin
Since this version of Joomla has not been released yet, this page can change anytime.
Heads-up, don't create a plugin as behaviour plugin because it's possible that this group get removed at some point.
Joomla 7 Compatibility Plugin
In Joomla! 7.0 the Compatibility Plugin introduced in Joomla! 6.0 will be replaced by a new plugin called "Behaviour - Backward Compatibility 7".
Despite the warning about the potential removal of the behaviour plugin type, this compatibility plugin still uses
this type to ensure it loads before any other plugin, as this is critical for backward compatibility functionality.
Detailed documentation
The compatibility plugin does contain some deprecated code which is only loaded when the plugin is enabled. In the removed backward incompatibility guide is documented in detail, what is moved to the plugin.