typedef void (*_qt_pixmap_cleanup_hook)(int);
typedef void (*_qt_pixmap_cleanup_hook_64)(qint64);
typedef void (*_qt_image_cleanup_hook)(int);
Q_GUI_EXPORT _qt_pixmap_cleanup_hook qt_pixmap_cleanup_hook = 0;
Q_GUI_EXPORT _qt_pixmap_cleanup_hook_64 qt_pixmap_cleanup_hook_64 = 0;
Q_GUI_EXPORT _qt_image_cleanup_hook qt_image_cleanup_hook = 0;
Q_GUI_EXPORT _qt_image_cleanup_hook_64 qt_image_cleanup_hook_64 = 0;
These symbols are exported, but not public (i.e. not declared in a header file), so one could argue either way as to whether this would be a source-compatible change. The symbols should be removed or the comment should be removed or updated to a Qt 6 to-do.