With the timeline fixed, Constantine, Charlie, and Zari rejoin the Legends aboard the Waverider.
Checking on the repaired timeline, Charlie learns that though Sara survived the unicorn, Nate, Ray, and Mick were killed by the fairy godmother the Legends encountered because Constantine still never joined the team.
By saving Desmond, Constantine never joined the Legends in the new timeline.
That means that he wasn’t at Woodstock to help the Legends defeat the unicorn, resulting in Sara’s death.