Volume doesn't matter most the time regarding indexes. 1.7 billion versus 2 billion volume isn't a big deal. The problem with a lot of shorts is they interpret every minor discrepancy as a bearish signal when it isn't. This is why they tend to lose money.
I keep a running average of daily volume over the last quarter. NQ, ES and YM volume today was between 77%-79% of the current average. Normally that is low enough to make it note worthy. However with the recent increase in volatility there was also a big increase in volume. Volatility and volume both seem to be slowly contracting. Meaning at least some of the "low" volume is simply reversion to the mean. Todays futures volumes were typical of the volumes before the recent volatility, and therefore aren't necessarily all that low. As someone who found little if any consistent use of volume for years I see why some ignore it. Prices moves with both contracting and below average volume do tend to be susceptible to significant retraces though. Of course it helps to know what "high" "normal" and "low" is. Personally I don't view the low volume by itself as a signal to fade a move but merely a warning that it may be running out of steam. Unfortunately the warning doesn't tell you whether the price move ends in two price bars or a dozen.