qtbase/src/widgets/graphicsview/qgraphicslayoutitem.h contains two to-do comments for Qt 5:

    virtual void updateGeometry();  //### rename to sizeHintChanged()
...
    // ###Qt5: Make automatic reparenting work regardless of item/object/widget type.
    QGraphicsItem *graphicsItem() const;

If either of these can be handled without breaking source-compatibility, this should be done for Qt 5.0.0. If not, the comments should be removed or changed to Qt 6 to-do's.