The independent Atlantic League and MLB entered into a partnership agreement a couple of years ago, with part of the agreement involving the Atlantic League adopting certain rule changes at MLB’s request that would allow the league to see how certain changes impact real life games. Several changes were implemented for the 2019 Atlantic League season, including banning mound visits and requiring pitchers to face at least three batters.
One change that was initially announced for 2019 was that the pitcher rubber on the pitcher’s mound would be moved back 24 inches. That change was then announced to be delayed until the second half of the 2020 season, but wasn’t actually implemented in 2020.