I wonder if you can get most of the benefit BYOB with a much simpler API:

    for await (const chunk of stream) {
        // process the chunk
        stream.returnChunk(chunk);
    }

(Lately I’ve been thinking that a really nice stream or receive API would return an object with a linear type so that you must consume it and possibly even return it. This would make it impossible to write code where task cancellation causes you to lose received data. Sadly, mainstream languages can’t do this directly.)