there have been some schemes that went way beyond just the players.
in the 1980's, the white sox actually used a light on their scoreboard to tell the batter if it was a fastball or breaking pitch. A member of the White Sox organization would sit in the manager's office and watch the camera that had a view of the catcher from center field and would turn the light on or off to indicate the pitch. Simple, effective and incredibly shady all wrapped into one. https://www.espn.com/mlb/columns/story?columnist=kurkjian_tim&id=1857661
The most damning of all may be the 1951 Giants that admitted to having a buzzer into the dugout to alert players to what type of pitch was coming.