The Streaming APIs (Millicast) use client-side ABR (referred to as “Simulcast” in the WebRTC world). This is currently supported in our browser-based broadcaster and in our OBS-WebRTC fork.
You may refer to this blog post if you want to learn more:
Millicast also enables users to stream RTMP MBR (Multi-Bitrate) for delivery via WebRTC Simulcast. This gives the streamer the ability to use their RTMP encoder to control their RTMP multi-bitrate renditions, and provide "adaptive bitrate" delivery in the Millicast web player using WebRTC Simulcast: