That's not gonna happen. Since Mac uses a different kernel, userspace and even their own proprietary graphics API, bringing Proton to Mac would take comparable effort to developing it for Linux. That's not worth it given the small amount of Apple gamers, which is self-inflicted thanks to many bad decisions Apple has made.

There's also the whole walled garden aspect. Getting away from Windows, just to lock yourself into the next closed ecosystem, is a terrible play. If Valve spent their efforts on this instead of focusing on improving Linux gaming, it would tarnish their image in many people's eyes.