They've been talking about it for years alongside with SSR streaming. Nothing concrete so far.
> The HTML output by this stream is exactly equal to what ReactDOMServer.renderToString would return.
I imagine this returns a stream as a convenience for Node servers that are streaming chunks of HTML. I don't think it means that React's SSR is rendering chunks of HTML in an async fashion and returning each chunk to the stream as soon as it's ready.
Also look at the last task of suspense:
> Implement a streaming server renderer