MLB and the MLBPA have agreed to continue in 2021 the implementation of certain rules changes adopted in 2020 in response to the COVID-19 pandemic, per multiple reports. Specifically, any doubleheaders that are played in 2021 will have both games be seven innings, rather than nine innings, and when games go into extra innings, a runner will be placed on second base to start each inning.
These rules are being adopted in anticipation of the likelihood that there will be games that have to be postponed due to positive COVID-19 tests, as happened quite a bit in 2020, which will necessitate playing doubleheaders to make up those games, and potentially eliminating some off days.