There was a time when pitchers enjoyed free rein to deface baseballs -- with mud, or streams of tobacco spittle, or maybe a belt buckle -- to alter aerodynamic properties. They did just about everything short of applying an ax at a time when each baseball itself could remain in play for an extended period. This is part of the reason this is referred to as the dead ball era.
But in 1920, Major League Baseball made a rule against the spitball and against foreign substances, with one caveat: The pitchers already in the big leagues who relied on the spitball could continue to do so. Spitballer Burleigh Grimes had made his debut in 1916 and was able to continue deploying the pitch through the end of his career in 1934.
Similarly, in 1970, MLB instituted a rule requiring hitters to wear helmets, but this also had a grandfather addendum: Any veteran who was already in the majors at the time the rule was put in place had the right to wear a cap instead of a helmet. In 1979, Red Sox catcher Bob Montgomery was the last player to bat without the protective headgear.
These precedents could be helpful for everyone to consider as commissioner Rob Manfred moves to implement new on-field rules, such as the 20-second pitch clock.