They didn't kill Yeoman Rand off. She was still alive during Star Trek VI: The Undiscovered Country, serving under Captain Sulu on the U.S.S. Excelsior. She was only in 4 episodes of the original TV series because the studio fired actress Grace Lee Whitney for being an irresponsible alcoholic and drug addict. She cleaned herself up in time to reprise her role as Rand in Star Trek: The Morion Picture, where she had been promoted to Transporter Chief.