During WWDC26 Keynote a second generation on-device model was announced with better speech generation capabilities.
Is there a new API available for developers to generate speech?
TTS Advanced Speech Generation: Expressive voices
Sorry, coming around late to cover unanswered questions. :)
The short answer is no. No new API has been released specific to that model. Though of course you still have the older existing speech synthesis APIs in AV Foundation https://developer.apple.com/documentation/avfoundation/speech-synthesis