Well they could just keep the wastegate closed in the recent years. That wouldn't make a difference to the new regs, besides the weight savings of not having to mount a wastegate plus pipes. In theory you can load the turbine by charging the battery and thus keep the revs low. Of course this will cause backpressure on the turbine.
I'm not even sure that keeping the wastegate and bypassing the turbine is the best strategy... As there's no more party mode available, maybe the manufacturers run the engine in (almost) sustained power mode. And then you want to maximise MGU-H charging so you keep the wastegate closed (or out of the car).