https://twitter.com/stephentotilo/status/1762576284817768457/
"NEW: Nintendo is suing the creators of popular Switch emulator Yuzu, saying their tech illegally circumvents Nintendo’s software encryption and facilitates piracy. Seeks damages for alleged violations and a shutdown of the emulator.
Notes 1 million copies of Tears of the Kingdom downloaded prior to game’s release; says Yuzu’s Patreon support doubled during that time. Basically arguing that that is proof that Yuzu’s business model helps piracy flourish."
It’s illegal if you don’t own the rom or decoded it with their keys. If you have the physical copy of the game, and a way to decrypt it, it’s not illegal to play the rom on an emulator.
So it’s not illegal, it becomes illegal when you don’t have the physical copy, or decode it with their stuff.