From The Greatest Minor League: A History of the Pacific Coast League, 1903-1957, by Dennis Snelling (McFarland, 2011), Kindle Loc. 1520-1526:
In late October 1919, American League president Ban Johnson proposed that “trick” deliveries, such as the spitball and shine ball, be declared illegal. At the major league meetings in February 1920, both the American and National leagues adopted the proposal, allowing a one-year grace period to pitchers identified by their teams as those relying on the spitter for their livelihood.” The Pacific Coast League followed suit, ruling that players currently in the league could continue to throw the spitter, but that pitchers new to the PCL could not. At the end of that first year, St. Louis Cardinals spitballer Bill Doak was among those asserting that banning himself and fellow spitballers from using their best pitch would likely end their careers.
Doak’s argument carried the day and the spitball remained a legal pitch for seventeen men during the remainder of their careers, including Ray Fisher, who did not play after 1920. This group continued as an endangered species of sorts until 1934, when Burleigh Grimes threw the last legal spitter in the major leagues.