Patch 5.1 Notes (Preliminary)

[Espritduo]

They literally announced it in the live letter that the Diadem will be reopen for the Ishgard restauration content.

Yes, which is why we all assumed there was nothing to worry about, and were looking forward to the new Diadem. Then the prelim path notes drop saying exploratory missions are being removed and the achievements will no longer be obtainable, which means the content behind it will be gone. Not changed. Gone. Here's the blurb from the notes so you can see how clear it is they are flat out removing the entire thing:

Exploratory missions have been removed.

The following adjustments have been made in accordance with the above:

Exploratory missions will no longer appear in the Timers window.
Exploratory missions will no longer appear as a duty in the Party Finder menu.
Achievements related to exploratory missions will no longer be obtainable.

Whatever the new Diadem is, it certainly looks like it will not let you obtain anything from the old Diadem ever again.

[Ultimatecalibur]

Yes, which is why we all assumed there was nothing to worry about, and were looking forward to the new Diadem. Then the prelim path notes drop saying exploratory missions are being removed and the achievements will no longer be obtainable, which means the content behind it will be gone. Not changed. Gone. Here's the blurb from the notes so you can see how clear it is they are flat out removing the entire thing:

Exploratory missions have been removed.

The following adjustments have been made in accordance with the above:

Exploratory missions will no longer appear in the Timers window.
Exploratory missions will no longer appear as a duty in the Party Finder menu.
Achievements related to exploratory missions will no longer be obtainable.

Whatever the new Diadem is, it certainly looks like it will not let you obtain anything from the old Diadem ever again.

It is likely they are changing the system to work like Eureka. Instead of trying to queue in with a Full Party you queue in solo. They are likely also removing the distinction between Trials of the Matron and Trials of the Fury. The Achievements being permanently removed (if any) are likely the "Feet/Hands/Head in the Clouds I-IV" and "Have a Haillenarte I-IV" achievements which are based on evaluation and completion as those systems will likely be removed replaced by other achievements.

Crown of Thorns I-V, Crowning Achievement I-III and Gem and the Hallowed Lands will likely return along with all current rewards once they finish the Diadem update in addition to new rewards and achievements.