Bungie is in the process of moving Destiny 2 from Battle.net over to Steam. The game is scheduled for September 17, 2019 release. Might be an idea for existing players to add the game to the wishlist so that migration will be quick.
Existing players will be able to migrate characters and DLC over to Steam so nobody needs to pay twice. New players can now preorder the game ahead of release.
Destiny 2 went free to play but all classes of players including those who bought the Shadowkeep, New Light, and Forsaken DLC are eligible for the migration. Once the migration is complete, the game is going to be removed from battle.net. A season pass seems to be the best bet for new players as this acquires 3 DLC packages which will give the most hours of gaming.
The 10 year contract with Activision for publishing has expired so Bungie has elected to self publish and leverage Steam where they hope they can make it work.