The two new "British" voices, which sound more natural than the existing two, are the only voices that have so far been added to iOS 17. It's not clear whether they were added in the third beta, or if they were included in previous betas, but they were uncovered prior to the latest updated third beta by MacRumors forum member MegaBlue.
In iOS 17, other Siri updates include an option to drop "Hey" from the "Hey Siri" invocation, the ability to get Siri to read web articles to you out loud, and the ability to interrupt Siri and make back-to-back requests without reactivation.
