The story, while tying into the larger narrative, can be enjoyed even by someone who is only slightly familiar with the overarching plot. Still, on its own, The Witch Queen is one of the best Destiny expansions, and its campaign offers more than enough content to basically act as its own game. Consequently, it is not the greatest pick for new players since they might feel lost unless they put in the effort to investigate the universe's overall lore. Now, the package is not backloaded with the previous expansions, so it only comes with the content associated with its name.
PS Plus Extra currently has Destiny 2: The Witch Queen, which is the game's sixth expansion.