In addition to giving streamers more control, this beta paves the way for future improvements such as new codec support, and experimenting with HEVC and AV1. AV1 is the next generation of video encoding technology — 40% more efficient than the current AVC standard — and it will improve visual quality, and reduce bandwidth.

Also: https://blogs.nvidia.com/blog/twitch-multiencode-av1-livestreaming/