Last I heard, WebReplay was not on an official roadmap (in the sense that we do not yet have a target date for shipping it in release). That said, I'm pretty sure that the macOS-only restriction is just because that's what the primary developer is using, and it's a higher priority to get the system-independent parts working first. If/when WebReplay is ready to ship, I can't imagine a world where support isn't eventually added for Windows/Linux (although it could be a phased rollout, like WebRender).