An interface for plugins and scripts to register custom logic to prevent the QGIS application from exiting was added. This allows plugins to determine whether it is safe for the application to exit. This has made it possible to allow plugins to determine whether there are unsaved changes which should be saved or discarded before allowing QGIS to exit, functionality which has already been implemented in the console script editor, preventing data loss by prompting users when there are unsaved changes rather than silently shutting down the application.
This feature was developed by Nyall Dawson