If you already own the game via Battle.net, make sure that you fill out the Steam migration info on the PC Move page. That way your owned licenses, and character data, will be transferred over to Steam on Oct 1st. Once the game has gone live on Steam, you’ll be clear to do a fresh install, and your save data will be waiting for you.
Destiny 2 was removed from Blizzard store in preparation for migration to Steam. The game will be available for download from Steam on Oct 1 (or potentially couple days before). Note that the base game, New Light, will be free to play and can be downloaded at any time after it's made available on Steam.