It is gone in Qt6. It was not documented as a class in Qt5.
In Qt5 QByteRef just included qbytearray.h.
#include <ctime> // for time, time_t, gmtime
#include <QByteArray> // for QByteArray, operator==
-#include <QByteRef> // for QByteRef
#include <QList> // for QList<>::iterator, QList
#include <QString> // for QString, operator+, operator<
#include <QThread> // for QThread
#include <random> // for mt19937
#include <QByteArray> // for QByteArray
-#include <QByteRef> // for QByteRef
#include <QDateTime> // for QDateTime
#include <QString> // for QString
#include <QThread> // for QThread