It's been available for quite awhile... (Destiny)

by Xenos , Shores of Time, Monday, August 13, 2018, 21:46 (2531 days ago) @ Phoenix_9286

Pre-order for just Forsaken has been available since the day it was announced:

https://www.microsoft.com/en-us/p/destiny-2/BQQHPTD44FL7

Unless you mean you want to get it and you don't own the previous expansions? If so this was made available for pre-order just a few weeks back:

https://www.microsoft.com/en-us/p/destiny-2-forsaken-legendary-collection/bz4k7n43rn97

Yes it comes with Destiny 2, which you already own, but you HAVE to own the previous expansions to play Forsaken (just like with Taken King) so buying that version is the best price.