Apple is offering tvOS 12 Golden Master for download for Apple TV tonight with build number 16J364 along with iOS 12 GM and watchOS 5 GM.
The Golden Master version, which is intended for developers and public testers, will be identical to the final version which will be released in a few days, more precisely on September 17. It will allow them to carry out a final series of stability tests.
To recover tvOS 12 Golden Master, go to Settings> System> Software updates on your Apple TV and let yourself be guided, if you had installed tvOS 12 beta.
Other recent Golden Master versions: