I read this in the patch notes for One Tamriel, and it got me worried:

Quests

Major quests have been updated to reward a green or blue-quality set piece.
Minor quests will continue to reward non-item green-quality set pieces.

Before One Tamriel, we had three types of items. Generic items, unique items and set items.

Generic items are things like "Ancestor silk gloves", with or without enchants.
Set items are items that gain additional powers when worn with items of the same type.

What I call here 'unique items' are typically quest rewards, or items dropped by world bosses, by named foes in delves and dungeons, and by Molag Bal's General summoned at Dark Anchors. They often have some in-game description making them a bit special. In my opinion, such descriptions were interesting, and in some case, transformed those items into genuine additions to the Elder Scrolls lore.

I fear that all those items will now be gone with One Tamriel, and I am a bit sad to lose in-game access to these tidbits of lore.

So, will it still be possible to get these items in the future?

I would prefer, for example, to get a set item as a quest reward in addition to, and not instead, the historic quest reward. There are still plenty occasions to get the set items elsewhere, by opening chests, killing foes, doing repeatable quests. The unique items, however, are accessible, well, in only one way.

If all the Unique items disappear, I fear Tamriel will start to look more generic, and display much less diversity.