I don't know where that came from, but none of the S.T.A.L.K.E.R. games make use of the DirectX 8.
1. Static lighting uses D3D9, as MSI AB/RTSS overlay shows.
2. Minimal system requirements state ATI Radeon 9600 and Nvidia GeForce 5700 which are DX9-class GPUs.
3. There are no references of d3d8 or d3dx8 in the binary files.
I checked some earlier S.T.A.L.K.E.R. development builds from 2005 and they also don't use D3D8. So it looks like DX8 was ditched from the game long before the release.