FreeScirocco pretty much summed it up nicely. But DX9 is still used not only for XP support, but also for Wine support. AFAIK, DX11 is still unstable with Wine, while DX9 games runs like a charm.
Temporal AA could be implemented by Scawen later too, if he ever implement motion blur (which is a must for a racing game imo), since he'll have a motion vector buffer to do temporal reconstruction.
But anyways, LFS doesn't have much high frequency detail in the distance, so temporal AA would not add much to the image stability compared to 2X super-sampling + 16x Anisotropic Filtering (+ MSAA 4x if you have post processing enabled) on a 1080p screen (which is doable easily on a modern GPU like a RX 480 or GTX 1060).