I've seen a reasonable amount, certainly at no less of a rate than I would normally expect for new reg changes(completely anecdotal of course!). I actually saw my first 73 plate in mid August, it was being kept at a dealership ready for the 1st of September as the owner had requested a 73 plate. I wasn't aware that it was possible to register a car "in the future" as it were
The date of a registration number’s activation on the DVLA database starts its liability to vehicle tax.
I took delivery my new car on 1 September. Nine days earlier, as a long existing customer, the dealership offered me a choice of five 73 registration numbers they had been allocated by DVLA. The dealership held on to my chosen one but only activated it with DVLA during the morning of 1 September. After I arrived at the dealership to check the car, including the 73 plates they had produced and fitted, I was then able to call my insurer to transfer my policy from the old to new car, and drove off early afternoon.
Although the dealership had no other 73s on that Friday (but others on the Saturday), I spotted a 73 on a different make of car in a nearby car park, but then saw no others until the Monday and very few during the month. So yes, the 73 seemed rare during September.