It seems in the recent Direct for Super Mario Maker 2, many fans have found potential proof for local multiplayer in the game as Nintendo themselves have yet to confirm if there will be any.
Check out some examples many fans have discovered:
In the eShop listing for the game, the number of local players is set from 1 to 4 players.
When you load up a course there is an icon which implies local 4 player multiplayer.
It seems that from this control option when creating a stage, only Mario and Luigi can be used but when playing a stage, 4 players can be used.
Check out this full analysis from Gamexplain about it below:
What do you think? Let us know in the comments.