https://blogs.unity3d.com/2017/07/10/deprecating-directx-9/

Due to diminishing hardware numbers and the loss of platform support from Microsoft, we have come to the conclusion that continued support of the DirectX 9 API is no longer feasible and is working against us when it comes to developing new features in Unity. It has become increasingly difficult to support and maintain feature parity across several DirectX API versions (9, 11, and 12).

With the rise of new graphics APIs, we feel that Unity continues to have excellent coverage of the hardware and API choices available now and in the future.