In the patch notes:
Non-Dungeon Bosses
Delve bosses will now drop a waist or feet set piece from the zone they are located in.
Overland group bosses drop head, chest, legs, or weapon set piece from the zone they are located in.
Public dungeon bosses drop a shoulder, hand, or weapon set piece from the zone they are located in.
Many of these Bosses have a small chance to drop a uniquely-named item, instead of the expected set piece. This uniquely-named item is also a part of one of the sets of the zone they are located in, but may not adhere to the dropped slot guidelines above.