https://www.polygon.com/2017/12/8/16752248/destiny-2-curse-of-osiris-issues-faction-rally

Destiny 2’s first expansion, Curse of Osiris, launched this week to a lukewarm reception, not least because it locks players out of content they could previously access if they don’t buy the add-on. Bungie is addressing at least one of the major complaints about that locked content: a specific achievement in vanilla Destiny 2.

A particular endgame feat — completing a Prestige raid or Prestige Nightfall strike — unlocks a PlayStation Network trophy or Xbox Live achievement, depending on which console you’re playing on. However, due to the power level increases for Prestige activities in Curse of Osiris, players who don’t own the expansion are now locked out of those exercises. In other words, the release of paid downloadable content is preventing people from being able to earn a trophy/achievement in the main game (and unlock its platinum trophy on PS4).