Following the announcement last July, Destiny 2 is now officially available on Steam.
Yesterday, Bungie brought Destiny 1 and 2 offline as part of the final migration and scheduled maintenance that took many hours. Now, the game just went live and along with its return, Bungie has also announced that Destiny 2 can now be downloaded on Steam for free.
Destiny 2 was initially released in 2017 and Battle.net was their official distributor. You can download Destiny 2 from their official Steam page.