In Path of Exile 3.28 (Mirage League), Harbinger has been removed as a core league mechanic. GGG has stated they intend to rework the mechanic for a future update, potentially as early as 3.29.0.

Harbinger Changes 3.28

Harbinger Removed as a Core League.

As turmoil grips the Atlas, the Harbingers appear to have departed Wraeclast, leaving behind only a cryptic message:

While the Beachhead lies quiet for now, perhaps one day they will return…

• Fracturing Orbs and Ancient Orbs are now able to drop as part of the core drop pool.
• Engineer's Orbs, Harbinger's Orbs, and Orbs of Horizons can no longer be obtained. Existing items will no longer function.
• Ancient Shards, Binding Shards, Engineer's Shards, Harbinger's Shards, and Horizon Shards can no longer be obtained. Existing items will be deleted upon logging in.
• The following Currency can no longer be obtained: Deregulation Scroll, Electroshock Scroll, Fragmentation Scroll, Haemocombustion Scroll, Specularity Scroll, and Time-light Scroll.
• The following Divination Cards have been temporarily disabled: The Journey, The Messenger and The Landing. Existing Divination Cards cannot be turned in currently.
• The following Unique Fragments can no longer be obtained: First Piece of Arcane, Second Piece of Arcane, Third Piece of Arcane, First Piece of Brutality, Second Piece of Brutality, Third Piece of Brutality, First Piece of Directions, Second Piece of Directions, Third Piece of Directions, First Piece of Focus, Second Piece of Focus, Third Piece of Focus, Fourth Piece of Focus, First Piece of Storms, Second Piece of Storms, Third Piece of Storms, First Piece of Time, and Second Piece of Time.
• The following Uniques can no longer be obtained: The Beachhead, Infused Beachhead, The Enmity Divine, The Flow Untethered, The Fracturing Spinner, The Rippling Thoughts, The Tempest's Binding, and The Unshattered Will.
• The following Uniques have been added to the core drop pool: The Immortal Will, The Shattered Divinity, The Surging Thoughts, The Tempest's Liberation, The Torrent's Reclamation, and The Yielding Mortality.
• All Harbinger related Atlas Passives have been removed.
• The Harbinger Scarab, Harbinger Scarab of Obelisks, Harbinger Scarab of Regency and Harbinger Scarab of Warhoards can no longer drop. These Scarabs will be converted to Veiled Scarabs upon logging in after the launch of 3.28.0.
• The Harrowing Carapaces Atlas Notable now grants Scarabs dropped in your Maps have 100% increased chance to be Divination Scarabs (instead of increased chance for Harbinger Scarabs).
• Added a new Atlas tree cluster Unexpected Stranger which causes your Maps to be inhabited by an additional Rogue Exile.
• Maps that had implicit modifiers for containing additional Harbingers instead now contain additional Tormented Spirits.
• Valdo Maps that added a Regal Harbinger to the area instead now add a Bismuth Ore Deposit to the area.
• Obscured Delirium Orbs and Foreboding Delirium Orbs can no longer be obtained. Existing Obscured Delirium Orbs will drop Breach Rings. Existing Foreboding Delirium Orbs have been converted to Fine Delirium Orbs.
• Foreboding Incubators no longer drop. Preexisting ones will instead grant drop Currency Items.
• Blueprints that had Harbinger Rewards now instead have Gold Rewards.
• Trinkets that had a chance to receive additional Harbinger items when opening a Reward Chest in a Heist instead now grant a chance to receive additional Gold.
• Runed Cavern Delve nodes no longer appear, existing nodes have been converted to Underground Stashes.
• Removed most cases of Currency Shards dropping throughout the game. Vendor recipes for Currency Shards are