Christian Slater is indeed credited in Str Trek VI: The Undiscovered Country." End credits, under Excelsior Crew listings as, IIRC, Starfleet Lieutenant. He is given the small credit as befits the brevity of the role. Perhaps those who say he is not credited mean he is not given an Opening Credit. Of course he isn’t; again, he had about three lines.
Same movie: I seem to recall that Rene Auberjonois did not receive credit as Colonel West.
Whoopi Goldberg, IIRC, is not credited anywhere in “Star Trek: Generations.”
There’s a slew of uncredited cameos in “Casanova’s Big Night.”