I got the same message (link expired) this morning. From the fine print at the bottom of the email about the reward: "Players must complete the Destiny 2: Forsaken main story by September 30, 2018 11:59PM PDT to unlock the free Destiny 2: Forsaken Original Soundtrack + Whisper of the Worm Bonus Tracks digital download." Nothing there about "must click on the link in the mail before X", and it that time is before the end of the challenge, it's bunk.