It turns out there’s a way to unlock 8-bit Luigi in Super Mario Bros. 35 after grinding through a bunch of the game, because it’s always the Year of Luigi somewhere.
It’s unclear who was the first person to discover Mario’s brother in the battle royale game, but players started to take note after Tetris speedrunner CombotronRobot shared a clip of Luigi appearing in world 1-1. The key appears to be holding L when selecting a stage up through when the new match begins, which results in the red-shirted, brown-overalled Mario being swapped out for his green-tinted brother:
According to CombotronRobot, you need to be star rank 100 in order to play as Luigi, although some players on the Switch subreddit have found that to be an insufficient condition. According to them you need to have unlocked and beaten every stage in the game as well, up through and including 8-4, Bowser’s final castle. It’s unclear if both of these are requirements, or just beating every stage, since most people who have reached rank 100 have also done the latter.
Luigi appears to control just like Mario, and his big form is the same as Mario with a fire flower powerup, but it’s still a neat little nod to the sibling who puts the “s” in Mario Bros.