Weren't steroids not even banned from the game till the early 2000s?
They were explicitly banned (barring a legal prescription) in 1991. Fay Vincent's memo said:
• The possession, sale, or use of any illegal drug or controlled substance by major league players and personnel is strictly prohibited. Those involved in the possession, sale, or use of any illegal drug or controlled substance are subject to discipline by the commissioner and risk permanent expulsion from the game.
• In addition to any discipline this office may impose, a club may also take action under applicable provisions of and special covenants to the uniform player's contract. This prohibition applies to all illegal drugs and controlled substances, including steroids or prescription drugs for which the individual in possession of the drug does not have a prescription.
• MLB recognizes that illegal drug use has become a national problem, and that some players and baseball personnel may fall victim to drugs. Baseball will not hesitate to permanently remove from the game those players and personnel who, despite our efforts to treat and rehabilitate, refuse to accept responsibility for the problem and continue to use illegal drugs. If any club covers up or otherwise fails to disclose to this office any information concerning drug use by a player, that club will be fined $250,000, the highest allowable amount under the Major League Agreement.
• MLB believes that its testing program is the most effective means available to deter and detect drug use. For admitted or detected drug users, testing will be a component of that individual's after-care program for the balance of his or her professional baseball career.
• This office will continue to search for positive and constructive methods of dealing with drug use. While baseball will attempt to treat and rehabilitate any player or personnel who falls victim to a drug problem, we will not hesitate to impose discipline, especially in those cases involving repeated offenses or refusals to participate in a recommended and appropriate course of treatment.
• If any club has a question about any aspect of the drug use program, please contact Louis Melendez, Associate Counsel, Major League Baseball Player Relations Committee.
Francis T. Vincent Jr.
Commissioner, Major League Baseball
CC: League Presidents
Player Relations Committee
Major League Baseball Players Association
Before that there was a general ban on the use of illegal substances that presumably applied to steroids, but I suppose some people could plead confusion or think that illegal substances meant something else (not a very tenable position, but I guess there's some wiggle room there. Post-1991, they'd been banned explicitly by name.