The Legend of Zelda: Majora's Mask just got an unofficial PC port

Yeah that's their intention.

Indeed, these decom projects do not include any of Nintendo's assets.

The code compiles 1:1 back into a unable ROM but isn't made just using a source code leak. It is reverse-engineered just like the SM64 decomp

For this to work, you’ll need a North American version of the game. Once you have it, drop it in the main menu and start playing.

Clever. I hope this protects the makers.

Like the Mario 64 Unreal Engine remake?

Edit: Unreal Engine, not Unity. My mistake. My point is that unless the project uses absolutely zero intellectual property, such as characters, stories, or franchise names, Nintendo will almost undoubtedly take ruthless interest.

It looks like the project is really careful not to include copyrighted materials in their distribution.

Source code automatically generated from copyrighted binary code is a derivative of copyrighted code, though. It's like taking a copyrighted book and running it through Google Translate and then clean up the sentences manually. You could be lucky that a publisher might not care about a translation into Icelandic but if you were to auto-translate a French version of a book into English and try to distribute it in the US, you'd probably get in trouble even if you leave out all graphic artwork.