Update: Bungie told GamesRadar that it has "no plans right now to have Destiny 2: The Collection available outside of Stadia," noting that it's "pretty easy to find ways to be content complete" without having to spring for an entirely new collection. The free Destiny 2: New Light includes all year one content, the Forsaken Compete Collection covers year two for $25, and year three starts with Shadowkeep for $35. After that, you can spring for Shadowkeep expansions as you like on an a la carte basis.
It looks like Destiny 2: The Collection, one of the Stadia's launch games, won't be limited only to Google's streaming platform. It launched yesterday on Stadia, but PEGI ratings suggest that it will also be coming to PC and consoles.
Since Destiny 2 is free, the appeal of the collected edition is the Forsaken and Shadowkeep expansions, which have to be purchased individually on PC. It's convenient, letting you dive into any part of the game you want without restrictions, but I also imagine it will be even more confusing as you figure out where to start.
When New Light launched, I wrote that it did a bad job of acclimating new players, and incredibly Bungie has since revealed that this was in part by design. It doesn't want you to play through the game chronologically, experiencing the campaigns as they were released and learning about the often dense lore. Instead you're meant to dive right into the new stuff without the benefit of the years of context veteran players have. It's very strange.
There's no release date or official word on the edition yet. On the PEGI website, they all have the same release date as the Stadia version, which was yesterday. Because it's available with a Stadia Pro subscription rather than being for sale, it's also not clear how it will be priced.