Live Relay is a new accessibility feature that's sure to be a boon for hearing and speech-impaired users. By combining text-to-speech and real-time transcription, it will allow you to conduct a phone call by just reading and replying to text messages on your screen.
While the Live Caption feature is for following along with audio playing from a media source, Live Relay takes after the call screening feature we saw on Pixel devices last year and extends it to the entirety of the phone call. Google explains the feature in this video, even hinting at plans for real-time translation down the line:
While this targets differently-abled users, it's also perfect for haters of phone calls (like yours truly), who'll be able to avoid even more phone conversations. Live Relay is still in the research phase, so we don't know when this will be rolling out.