Apple today released tvOS 16.3 RC preview update for testing, specific version number (20K650).
Developers can download the tvOS 16.3 Beta by using Xcode to download a configuration file to Apple TV.
The tvOS updates are usually small, focusing on internal bug fixes and improvements rather than obvious outbound changes. No obvious new features have been found in Apple tvOS 16.3 yet.
Apple has shared some information about the tvOS release in its tvOS support documentation, which is updated with each tvOS release but does not provide details about the Beta version.