The QEvent code contains the following Qt 5 to-do comments:
class Q_GUI_EXPORT QUpdateLaterEvent : public QEvent
As source-incompatibile changes are no longer permitted for Qt 5, this comment must either be removed or changed to a Qt 6 to-do.
class QKeyEventEx : public QKeyEvent
As this is in a private header, source-conpatibility is not an issue, so the comment can be actioned (or removed, or changed to a Qt 6 to-do).
QDropEvent::QDropEvent(const QPointF& pos, Qt::DropActions actions, const QMimeData *data,
Qt::MouseButtons buttons, Qt::KeyboardModifiers modifiers, Type type)
: QEvent(type), p(pos), mouseState(buttons),
default_action = Qt::CopyAction; drop_action = default_action;
This comment does not appear to impact source or binary-compatibility so it probably does not have to be actioned for a major release (in which the comment should no longer say "Qt5".