I have said that I do not think that anyone would design a shock wave into an engineering device.
In section 3 I discussed the use of convergent-divergent nozzles in steam turbines. The nozzle is part of an engineering device comprising the ring of nozzles and a ring of moving blades. All the velocities involved are high and we have seen that the steam speed is likely to be around Mach 1. It is not easy to find the best proportions for the nozzle and the moving blades to get all these velocities sorted out to give a high efficiency for the extraction of energy from the steam. It may be that it would be desirable to have steam entering the moving blades at supersonic speed but this must be balanced against the consequences of having the inevitable shock waves form in the blading. Energy is lost in these oblique shock waves. So there is an incentive to avoid supersonic flow altogether. There is little likelihood that conditions leading to plane shock waves in the nozzles are necessary.
The rocket nozzle is rather different because the creation of a jet having a high Mach number also creates forces on the nozzle to give thrust. This being so the question of the existence and consequences of having a shock wave in the divergent cone must be relevant.
In figure 13-11 of the rocket engine I just drew a notional pressure distribution through the nozzle. Now I have to look at the discharge from the nozzle to see how it might affect the thrust
A rocket engine used to lift a space craft into orbit will start off discharging to atmospheric pressure and end up discharging to a pressure that is close to a vacuum. In graph 13-7 the pressure at exit is well below atmospheric pressure but atmospheric pressure is well below the back-pressure needed to cause a shock wave to form in the nozzle. What does change as the pressure surrounding the nozzle decreases is the net force on the rocket increases simply because the external pressure on outside of the rocket motor decreases.