When closed beta tests occurred, access required:
There is no evidence of a persistent closed beta client available for download at this time. mario multiverse closed beta download
⚠️ Important: This is a closed beta. You need an authorized key to unlock the full build. Unsigned builds will not connect to the test servers. When closed beta tests occurred, access required:
Historically, Nintendo is ruthless. They shut down AM2R and Pokémon Uranium. However, Team Paradox has built a legal shield: Mario Multiverse requires a legally obtained Super Mario World ROM to generate textures. The download link only contains the engine and code; you must supply your own smw.smc file (which you legally own, of course) in the /Assets folder on first boot. There is no evidence of a persistent closed
This is why the Mario Multiverse Closed Beta download is a gray area—the developers aren't distributing Nintendo IP, just a tool to play with it. Whether that holds up in court remains to be seen.