Roblox’s 3D grass feature was announced on November 10, 2019, and Roblox said it would reach all clients a week or two later. A developer forum follow- up on November 30, 2019 said it was still being tuned and planned to ship before the end of that year.

What that means

The feature was first shown in Studio in mid-November 2019, then rolled out to clients shortly after. So the safest answer is: late 2019 , with the public announcement on November 10, 2019.

Quick note

If you meant a different “3D grass” in a specific Roblox game rather than the engine’s terrain grass, the date could be different.