Back in 2020 when the league was trying to get the pandemic-shortened season underway, one of the newly-implemented rules was the adoption of the 7-inning doubleheader. See, to try to squeeze in as many games as possible, team were to play more doubleheaders than usual. To keep the teams from getting too burned out playing that many games, though, the doubleheaders were shortened to 7 innings. It was originally meant to be a rule only for 2020, but MLB kept the rule in place for 2021.
Fast forward until April 25 when Madison Bumgarner pitched a complete, but shortened, 7-inning game and allowed no hits.