It's been years now. It's either going to happen very soon or its not going to happen at all. I had hoped that the Microsoft merge would have helped speed it along.You have a point with libraries. I'm not at all familiar with them, so I don't know how a reasonably-well-designed game would use them or how difficult they would be to decompile, modify, and replace.
In any case, I think we all know Mojang's stance on an actual modding API. How many years have they been promising to create one?