Destiny 2 is now availale to preload on Steam. Expect a 79.1 GB download which is probably the reason for the week before the formal Steam launch. The game needs 105 GB of storage once its unpacked.
You can get Shadowkeep from this link.
By comparison, Elder Scrolls Online is slightly larger at 85.7 GB to download which suggests more and more games need to be distributed on Blu Ray. Blank BDR-100 disks are not that expensive for backups of games. Bungie was on Battle.net before realizing Steam has more to offer. Many developers are now acting as their own publisher and Bungie decided that Steam was worth the cost.
Now that game is on Steam it’s easier to move to a SSD when there is available space. Origin has now added that feature but other loaders are not yet as sophisticated. The game, after downloading, remains in the download table until release day. This is typical for may preloads as there are new updates released on release day. Unpacking the game on release day was sluggish due to the huge size of the game.