You are right again, stickboy.
But don't you think "described" is too strong a word? My copy of changelist.htm just says, "Removed HSIAdjust()." Two words, plain and simple. Hardly what I would call a "description". Really just a "notification." And not even close to an "explanation".
Anyway my problem has been solved for the moment, with your help, and I'm encoding my video right now. Your suggestion to use the VirtualDub filter import script did the trick.
Thanks also for checking the HSIAdjust function. So are we to conclude that control of RGB saturation has
never been supported in any version of Avisynth? (I'm shocked.)