It's now confirmed that Destiny 2 on Google Stadia, Google's upcoming cloud-based game streaming service, cannot cross-play with other platforms, including PC. Google Stadia will be limited to its own "ecosystem," meaning Destiny 2 players on Stadia can only play with other Stadia players. It doesn't matter if Stadia users are using keyboard and mouse, controllers, or otherwise, they'll be matched limited solely to multiplayer with other Stadia Destiny 2 players.
The news stems from Google Stadia's FAQ regarding its Stadia Founder's Edition product package. The Stadia Founder's Edition packages a controller, three months of Stadia Pro subscription, and also the complete Destiny 2 including all post-launch content up to and including this fall's Shadowkeep expansion. The specific question answered is, "Will Stadia feature cross-play with Steam and other platforms?" and the answer is no, "Stadia players will only be able to play with other Stadia players."
No reasoning or justification for the decision is provided for the decision to segregate the Stadia community for Destiny 2, though a logical explanation can be speculated. Given the nature of Google Stadia as a cloud service, it's unlikely that it will feature a similar game licensing and account system. Someone logging into Destiny 2 from Stadia won't necessarily be accessing their personal copy of the game, they'll be accessing a pool of licenses that Google Stadia has reserved for its players.
As impressive as multiplayer technology is these days, even with Bungie developing cross-save technology across all platforms, some knots would simply take too much time and effort to unravel. Given that Bungie has managed to cross-saves working, though, perhaps there will be hope for cross-platform between Google Stadia and at the very least PC in the future.
Just because it might be understandable why cross-play between Google Stadia and other platforms isn't feasible doesn't mean it's going to go ever well. Many Destiny 2 players on PC were excited about the possibility of being able to log in to Google Stadia from a laptop or tablet while away from their desktop and still play with their usual Fireteam. And many prospective Google Stadia subscribers were interested in large part because it'd be an easy way of joining the already active PC Destiny 2 community. Google Stadia must have a lot of confidence to believe it can maintain its own multiplayer ecosystems.
Destiny 2 is available now on PC, PS4, and Xbox One, with plans to launch on Google Stadia later this year.