YouTube. 

It was possible to cast from YouTube Music’s predecessor, Google Play Music, although I think it was only from Android devices.  (For iOS, Airplay is often an option. ) It was Google’s decision to replace GPM so ultimately they are responsible.

There is no way that Sonos can implement direct casting unilaterally.  It requires Google to suggest it and both companies to work co-operatively to make it happen.  Whether that process is under way, or planned, or totally ruled out, I don’t imagine anyone on here could tell you.

Thanks. Has anyone heard Google or Sonos ever comment on this issue? 

As a rule, Sonos never comments on features they are working on, I would guess to maintain a competitive advantage in the marketplace. 

Given that they’re embroiled in a lawsuit at this moment, or at least last time I checked, I’d be surprised if there was any desire on Google’s part. They’re the ones being sued by Sonos for stealing tech. 

https://www.theverge.com/2020/9/29/21492671/sonos-google-patents-infringement-lawsuit-wireless-audio