Twitch is currently testing Enhanced Broadcasting, which is how they will enable HEVC and AV1 streams later and currently improves quality by not relying on their transcoding.

Currently, I'm enabling enhanced broadcasting by sending NDI from vMix to OBS, but this setup runs at 90% CPU usage and bumps up right against the NVENC session limit (and has a noticeable generation quality loss from the NDI compression).

Having native support for this in vMix would be a huge upgrade. If I'm not mistaken, vMix already supports RTMP+ for sending HEVC and AV1 to YouTube, so hopefully y'all can work with Twitch to enable this.