Hackers and dataminers have discovered what the debug menu does in Splatoon 2.
According to OatmealDome, a prominent dataminer in the Splatoon 2 community, the debug menu allows players to set up “super private battles”.
Super Private Battles allow players to battle against each other via local wireless and/or LAN. Weapons, abilities, and gear could be fully customized in custom Super Private Battles.
The only way to access Super Private Battles is through installing a patch and the Global Testfire. More details can be found here.