Several weeks ago Activision announced live at BlizzCon that Destiny 2 would be free on PC by way of its Blizzard Launcher. Well the time for the promo has almost passed (it expires on November 18), so if you want it, you have another day to go grab it. If you don't have a Battle.net account yet you'll need to sign up for SMS Protection first (the instructions are here).
As a reminder, the release of the "free game" is a calculated marketing ploy. Most of the current content is tied to paid DLCs (including the new massive Forsaken expansion and all of the upcoming other season pass add-ons), so you're mainly just getting a limited version of Destiny 2 for free.
Still, you get to work your way through the campaign and a few other modes for nothing: so you may as well just click the "get it" button and save it for a rainy day.