Interesting findings in the SMK source code so far :

➡️There were at some point 5 Battle maps listed
➡️Unused items in the final game (1UP shroom, an "Outside Ghost" and/or an item listed as "The enemy's jump trigger!" (prolly some sort of forced feather for the opponent)

⬇️ ➡️ Koopa & Toad were initially supposed to be faster than Yoshi & Peach, but eventually they were made slower.
➡️Rainbow Road was added very late in the game, previously there was a third Choco Island track.
➡️Seems like NBT are referred as Drift Dash in the game code

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