Update: Destiny general manager Mark Noseworthy has clarified on Twitter that there is "no policy or technological barrier" preventing crossplay with Stadia, and that if and when Bungie implements it, it will aim to bring it to all platforms.
We’ve already mentioned that we’d love to bring cross play to Destiny 2. There’s no policy or technological barrier preventing us from including Stadia. We’d be looking to include every platform. This year though, we’re focused on delivering cross save to all platforms. https://t.co/uSVLsBZt0SJune 16, 2019
"Stadia players will only be able to play with other Stadia players", Bungie said in an FAQ published this week. It didn't explain the decision further.
I'm interested to see whether other multiplayer games coming to Stadia, such as The Division 2 and Elder Scrolls Online, take the same approach. Google's Stadia service will stream a PC version of a game to your TV, desktop or laptop, so you'd expect fewer barriers to crossplay than between PC and consoles. But there may be advantages to keeping the "ecosystem" within Stadia that we don't have all the details on.
Destiny 2's presence on Stadia was just one of many announcements Bungie made last week about the shooter's future. It's going free-to-play, moving to Steam, and getting a major expansion called Shadowkeep. It's also introducing cross-saves between different platforms, so you'll be able to transfer your progress between your PC and Stadia versions, if you like.
Thanks, Game Informer.