I think Hellbringer is, likely, correct with respect to when the rear wing became standard. That isn't to say a wingless car could not be ordered, as was apparently the case with Brian Kallaher's car. At least from the majority of responses in this thread, it appears most factory wingless cars came very early in the MKIV's five-year production cycle, with the rear wing becoming standard at some point, while wingless cars could still be ordered from the factory when buying new from a dealer.
I don't know how old Tim is, and I'm not questioning Kallaher's representations, but the gestation period for the MKIV Supra was from 1989 to late 1992. Toyota had been stung by the criticism regarding the weight of the MKIII Supra, so significant effort was put into making the MKIV significantly lighter than the MKIII. I wrote a paper about this many years ago.
Toyota conducted a total of 956 meetings during the car's '89-'92 development with the sole objective of taking weight out of the car. This resulted in hollow fiber carpets, a hollow rear spoiler, bolts with hollowed out heads, aluminum hoods, aluminum targas and many other detailed items. This was, by all measures, a highly successful effort when you take into account that the MKIV came with two turbos, as opposed to the MKIII's one, a 6MT as opposed to a 5MT, 4/2 piston brakes as opposed to 1/1, dual air bags, as opposed to one, and 17-inch wheels as opposed to 16-inch wheels.
All of this information was public and you did not have to be a designer of the car to be aware of it. Certainly, being a designer would make someone privy to information not publicly available, but how cars are configured when they are sold is not something that falls within the area of responsibility of the vehicle's designers. That is, generally, the province of the dealer purchasing the car from the factory, and a dealer is going to want to have the flexibility to align his vehicle purchases with the needs of his customers wishing to buy the affected vehicle, the MKIV Supra in the instant situation.
Initial dealer reservations regarding the rear wing were overtaken by the popularity of the rear wing, and I believe those of us that have been around for a while remember when wingless cars weren't considered as desirable as cars with a rear spoiler. So, some wingless cars, like Pasadena Turbo's, were converted. Having said all this, it appears to me, at least, that wingless cars have enjoyed a renaissance of sorts just like the color Anthracite Metallic (and Baltic Blue to a lesser degree), which has evolved from monkey shit brown to one of the more desirable colors in the MKIV's palette. Such changes in tastes are not surprising when one considers our cars range in age from 19-24 years of age.
P.S. Also, please remember the price of the MKIV Supra was significantly higher than the MKIII Supra, and the huge apples to apples price increases fed dealer anxiety along with initial concerns about the acceptability of the car's rear wing.
Ken.