There is no "all we have today is drums, so we will build with drums even though the car is designed for discs". If there aren't any of the correct part to go on the car, the line goes down and the supplier gets beat about the face and balls until parts show up (and they get back charged for plant downtime to boot). Besides, the design for a disc brake knuckle and drum brake knuckle are different enough that you couldn't just swap back and forth. Different half shafts would be needed. Brake bias would be all fucked up, different master cylinders would be needed, etc. Can't be done easily.

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My dad and I were talking earlier. Backstory: I tagged along with my mom to the dealer to get the oil changed on  her Buick Enclave. Turns out the rear brakes needed replacement, so dad signed off on it.  He didn't know at the time it was 'rear brakes' as my mom just said brakes.

Today he asked me if it were just the rears and I said yes.  He said "I wonder why they did that" and I told him the service girl said it was down to 2mm left.

Then my dad asked me if the Enclave has rear drums. I Googled to confirm what I originally though and I said no, all disc.  And he said "Well maybe it has rear drums, they just put on whatever they had at the factory."

I don't think so, but my dad does.  What I know from following years of automotive research is that manufacturers generally plan out their vehicles years in advance.  Of course it would have four wheel disc, all the Enclave's competitors do.  When you're buying a $50k crossover, why would you want one with drum brakes?  While manufacturers would do mid-cycle changes or year-to-year changes they wouldn't put on different type of brakes than the next just because "that's all they had."

The only reason my dad said they may have "put on whatever" is because back in the day, my dad owned a 1988 Chevrolet truck that was a work truck built on the R(Recreational Vehicle) chassis. The brakes for the vehicle had to be 1983 brakes.  That makes more sense, because GM recreational vehicles and stuff like box vans are generally known to just be mishmoshes of parts bin.

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GM made the "old body style(1973-1987)" pickup for a couple years while building the "New body style(1988-1998)". AFAIK, these were all built in the 1 Ton version, mostly for fleet sales.
Much like they did for the Suburban and the Blazer(V), the R series is essentially the same as a K series from 73-87.

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So the dealer charged y'all to do a drum brak conversion?

Or did they just replace the pads but your dad thinks it was drum shoes?