It turns out that Luigi is in Super Mario Bros. 35! However, it appears that only players who have reached Level 100 will be able to play as him. More information regarding how to play as Luigi is still coming out, but it appears that eventually playing as Luigi is as simple as holding down the L button during the match pairing screen!
IMPORTANT INFORMATION:— Combo @ Open Commissions (@CombotronRobot) October 20, 2020
>I'm not sure of the timing for holding L, I held it during this whole video until the match began
>It also seems like you will need to be at a star rank (level 100+) to actually do this too