Wikipedia has this to say on the history of banning spitball:
The spitball was banned in two stages. In the winter of 1919–1920, managers voted to partially ban the spitball, allowing each team to designate at most two pitchers who would be permitted to legally throw spitballs. Then, following the 1920 season, the spitball was banned leaguewide, except for existing spitballers who were grandfathered in and allowed to keep throwing the pitch legally until they retired.
The first stage looks highly surprising, but somewhat fair. It's a way to allow some spitballs in the game but not all spitballs.
The second stage puzzles me. Did this decision lead to inequality of rules between players and teams? Was such inequality real, and seen as a problem? Were there any considerations that made banning spitball for everybody impractical?