In an effort to reduce the number of pitching changes and, in turn, cut down the average time per game, MLB instituted a rule change that requires pitchers to either face a minimum of three batters in an appearance or pitch to the end of a half-inning, with exceptions for injuries and illnesses.
If a pitcher faces one batter to end an inning, he may be removed, but if he is brought back for a second inning, he must still face two more batters for a total of three.
Previously, Rule 5.10(f) stated that the starting pitcher must pitch to one batter until that batter was put out or reached base, and Rule 5.10(g) stated that any reliever must pitch to one batter until that batter was put out or reached base, or the offensive team was put out, with exceptions for injuries and illnesses. These rules were in effect through the end of the 2019 season.
The 2020 change will likely cause a decline in the number of specialists -- such as the "LOOGY" (lefty one-out guy) -- who are primarily brought in for certain matchups.