Harbinger was released alongside 3.0.0 and was played by more people than any league we have made before.
When looking at the league as a potential permanent addition to Path of Exile, there are a few things we need to consider. The game now has a lot of additional encounters, so our first question is, "do Harbinger encounters provide something the core game needs?" While we think the encounters are enjoyable, they don't provide something the core game needs on their own. Therefore, we don't feel a need to make it part of the permanent composition of Path of Exile.
Harbinger will be available from Zana as a league option in the future, though not in 3.1.0.
Harbinger provided a number of new currency items, as well as shards for existing currency items.
The two map rerolling currency items (Orb of Horizons and Harbinger's Orbs) are not something we want to keep in the core game. While they provided players a lot of flexibility in mapping, we felt it took a lot away from unlocking the Atlas, and made jumping up map tiers too easy. They also do not fit well with content coming in 3.1.0. These will not be added to the core game.
The Ancient Orb was not of great interest, except in cases where it provided the most value, and shortcut access to the most chase items. We feel the shortcut it provides is too great, therefore it will not be added to the core game.
The Orb of Annulment provides great risk to high end crafting, but also the potential for great results while adding a new wrinkle in the path to great items. We knew that this item had great risk associated with adding it, especially when used in Standard league with Eternal Orbs. It turned out to be a fine addition to the game. Players had many great moments, and also some terrible results, giving it a similar feel to the Vaal Orb in crafting. Its ability to sometimes salvage an almost-great item makes it an appealing addition to the core game. We will add this as a currency drop in the core game.
The Orb of Binding and the Engineer's Orb did not resonate with players, and do not fill great enough need to be added to the core game at this stage.
The new Currency Shards will not be added to the core game. The Currency Tab slots for Harbinger items: Orbs of Annulment, Annulment Shards, Chaos Shards, Regal Shards, Exalted Shards and Mirror Shards, will be left as they currently are. We may in the future make them invisible if you don't have any items of that type in the tab.
None of the unique items from Harbinger will be added to the core game, and will only be available when the league is available from Zana. The Flow Untethered and The Tempest's Binding did have features that were popular with players, and you may see the key features of these items appear in another way in 3.1.0.
Unique items from Harbinger have not been identified as needing any rebalancing. We will adjust the relative chances of unique shards to keep the items accessible when the Harbinger League is available from Zana.
As well as the mechanics, Harbinger League had a strong lore basis, and had an associated language that provided hints to the future. We were very happy to see some players put a great deal of effort into decoding this language, and uncovering what may be coming. While it does signal what will come up in the future, this will not be seen directly in 3.1.0. It's something to keep an eye out for in the longer term.