https://www.pcgamer.com/thanks-to-a-bug-destiny-2-forsaken-items-are-dropping-before-the-expansion-is-out/

The Destiny 2: Forsaken pre-week festivities kicked off yesterday, and in predictably fine form it has not gone entirely smoothly. As noted in this Reddit thread, weapons and armor from the expansion have already begun to drop, almost a full week before they were supposed to. 

It's apparently quite easy to get your hands on Forsaken equipment: According to this Houndish video, the stuff is dropping as a reward for completing the Flashpoint milestone reward on Mars. Whether you'll actually be able to equip it is dependent on your character level—you might have to do a little item-and-character juggling if you want to get on the new triggers right away—but the stats and perks are all intact and functioning.