It's now marked as internal. Which means it's not intended for public usage anymore. To use it you need to add into your .pro
QT += 3dcore-private
and include:
#include <Qt3DCore/private/qcircularbuffer_p.h>
and that's it:
Qt3DCore::QCircularBuffer<int> b {1,2,3,4};
qDebug() << b.last();