Limited support
This feature is only present in Portal 2: Community Edition and Portal: Revolution.
Caution:

FadeDistance is an internal entity and should not be used on its own.

BASE FadeDistance

KeyValues

Start Fade Dist <float> = -1

Distance at which the prop starts to fade (<0 = use fademaxdist).

End Fade Dist <float> = 0

Maximum distance at which the prop is visible (0 = don't fade out).

Fade Scale <float> = 1

If you specify a fade in the worldspawn, or if the engine is running under low end/medium end, then the engine will forcibly fade out props even if fademindist/fademaxdist isn't specified. This scale factor gives you some control over the fade. Using 0 here turns off the forcible fades. Numbers smaller than 1 cause the prop to fade out at further distances, and greater than 1 cause it to fade out at closer distances.