In 2017 folks crafted Super Mario 64 Online (referred to as "SM64 Online"), a project that allowed folks to play with 24 players in a modded version of the N64 classic. It was promptly taken down after Nintendo got wind of it, and now it's coming back as the not so conspicious Net64 2.0.
As you might have guessed it's a 2.0 version of the game, and as a result of the new release, Sonic characters are in. Okay! That's cool, I get that. This is a new project under new management it seems (a user named Tarnadas), and allows for PVP events like CTF. The creators surmise that the project will "stay up for a little while longer [than the original]."
To all of you saying that "the press is ruining this low key production," the creators put a public YouTube video up of the project that now has close to 100,000 hits. The cat is out of the bag.
Super Mario 64 Online Is Back From the Dead, For Now [Kotaku]
via destructoid https://www.destructoid.com/fan-project-super-mario-64-online-adds-sonic-characters-502916.phtml