As reported by TechPowerUp, this feature got added with the release of last week's GeForce 384.76 WHQL drivers.
Whether this still makes sense as most of the Fermi chips are at least six to seven years old is a question we're not going to answer, but TechPowerUp speculates Windows 10's Creators Update may have been NVIDIA's motivation to finally live up to its promises:
The support appears to be sufficient to run today's Direct3D feature-level 12_0 games or applications, and completes WDDM 2.2 compliance for GeForce "Fermi" graphics cards on Windows 10 Creators Update (version 1703), which could be NVIDIA's motivation for extending DirectX 12 support to these 5+ year old chips.