Ah... you mean the Alt-D option.
It seems to have gone. You have to preset it from the graphics
details menu.
Shame really. I've run quite a lot of races with low trackside
options for the first lap (while there's a lot of cars to display)
and then switch it back on for the rest of the race.
Same with the option to turn off sky texture and track texture,
and also verge texture.
Gone. (completely)