It has just come to my attention that in addition to Home Sharing, the YouTube streaming app has also retained the ability to stream 5.1 audio since the tvOS 18.0 update was released. Because I have also installed the tvOS Public Beta 18.1 in an attempt to find a remedy, I don't know whether the 18.1 Public Beta release is required to allow the YouTube app to stream 5.1. Here's an identifier for a YouTube video that can be used to demonstrate: v=ijSYp6N5Yrs