I looked up the specs online and sure enough, one of the variations of the Impala that year (with a particular engine and sport package) intentionally had no ABS. I couldn't find the rationale for the exclusion, though.
Likely a former rental car; GM has been known to do up cheaper specials for fleets that are missing equipment which any consumer-bound model would have standard.
Anything's possible, I suppose, but according to the documentation I obtained independently, the car was privately owned by only one prior owner, a middle-aged single guy (in the neighborhood, as it turns out) who routinely purchases new cars, owns them for two years, and trades them in for a new another new car. (Expensive as all get-out, but whatever floats your boat if you can afford it…)
I just looked it up; I had the LS base model. All the Impalas that year had ABS except the LS base model. (Who knew?) Of course, as you say, GM may have manufactured the LS base model mostly for the rental market. I'll go with you on that.
Anyway, I guess we've pretty much exhausted what anyone may have *possibly* wanted to know about some car I once owned and the brakes it did or didn't have.