Released in 2006, Flight Simulator X introduced unofficial support for DirectX 10, to enable more reflection effects, change the water look, and to increase the number of frames per second. It remained as a “Preview” mode in the display settings, as it may appear some display issues and overall if you use third party addons : flickering textures, missing textures on AI aircraft, custom ground or markings etc..
Anyway, there are DirectX 10 lovers in the FSX community, and Steve Parsons alias SteveFX has made a lot to limit or fix their issues. First, with his freeware modules still found here. Now, he even sells a complete solution DX 10 Scenery Fixer. Visit his website.