Destiny 2's dev team apologized for locking the players out of endgame content. They explained how they followed a strategy that worked in the original Destiny. A hotfix that'll be deployed soon will aim to remedy the situation.
"In Destiny 1, as your character grew more powerful throughout each expansion, some of our best content, like Vault of Glass, was left behind and lost its relevance for players. We wanted a better solution for Destiny 2, where all of our Endgame activities could stay relevant as each Expansion causes your Guardian to grow more and more powerful" says the blog post. I might have been wrong.
With the separation of "Normal" and "Prestige" mode, the team had two goals in mind. The Leviathan Raid and Nightfall in Normal mode would always stay at a Power level that was accessible to all players. The Prestige difficulty, on the other hand, would always raise to the latest Power cap.
"It could be the pinnacle of challenge, with the most prestigious rewards, but it would therefore require you to own the latest Expansion and be at the new Power cap."
As for the Seasonal, time-limited PvP playlists: they are meant to evolve each Season, as well as utilise new maps. Meaning that it would be required of you to own the latest content. "To play the latest season of Iron Banner or Trials, and earn the new rewards, players would need to own Curse of Osiris".BungieDestiny 2
That was the plan. The reality check came in the form of some common sense: "The Prestige Raid was a novel experience that players value, even if they don’t own Curse of Osiris, and it was a mistake to move that experience out of reach. Throughout the lifetime of the Destiny Franchise, Trials has always required that players owned the latest Expansion. However, for Destiny 2, Trials of The Nine launched as part of the main game, so it’s not right for us to remove access to it."
The team went and made matters worse by overlooking "the fact that both of these mistakes disabled Trophies and Achievements for Destiny 2". To remedy the situation, Destiny Dev team will deploy a hotfix that'll, among other things, bring the Prestige Leviathan Raid back to down to Power 300, and its rewards will drop down to match the new Power Level. All players will regain access to the Prestige Raid.