Google says it was 2022:
"The Hoard Mimic was added to Gems of War on March 9, 2022, as part of the 6.2 Update: The Secret Hoard. It is a rare Mythic troop that can appear after defeating a Mimic in a Boss Chest battle, primarily during Explore mode. [1, 2, 3, 4, 5]
- Update: 6.2 (“The Secret Hoard”)
- Release Date: March 9, 2022
- Context: Added alongside the Adventurer’s Path feature.
- How to Find: Dropped from Hoard Mimic battles, which are triggered randomly after defeating a regular Mimic in Explore mode. [1, 2, 3]
The troop is considered very rare, and many players reported significant grinding to obtain it after its release in early 2022."