For context, The Big House is a Smash Supermajor that runs annually. Obviously was planned to be online this year due to COVID and all. As for Slippi, they've implemented rollback netcode and automatic replays for the Melee, it's basically an extension off of a dolphin build if I remember correctly. Regardless, Nintendo doesn't seem too happy with this fact, even if this is pretty much the gold standard for online Melee nowadays.
Bonus points to Nintendo for shutting down the Smash Ultimate tournament as part of this?
To be fair, they're within their legal rights, but man, did they learn nothing from EVO2013 when they tried to C&D that?