Prior to tvOS 11, public betas were limited to iOS and macOS software likely due to the slightly more complicated tvOS beta installation process, but now public beta testers can access all software platforms with the exception of watchOS thanks to new installation procedures.
The tvOS 11 public beta can be obtained by going to the Settings app on the Apple TV and navigating to the Software Updates section under "System." "Get Public Beta Updates" will need to be toggled on, and once it is, the Apple TV will download the beta software.
Compared to iOS 11, macOS High Sierra, and watchOS 4, tvOS 11 is a minor update that brings few changes to the tvOS operating system. It's minor enough that it received no time on stage at the Worldwide Developers Conference aside from a mention of an Amazon Prime Video app coming to the Apple TV this fall.
According to Apple's release notes, tvOS 11 introduces automatic light/dark appearance switching based on local time, Home screen syncing options for syncing content between two or more Apple TVs in a household, new background modes and notification support, plus new tools for developers and improvements to Mobile Device Management.
Additional tvOS 11 features may be unveiled ahead of the operating system's public release, which is expected to be in the fall alongside other software updates.