I use them all the time for predicting bottoms/ tops and I've observed over countless hours of studying charts that it's the X.618 (1.618, 2.618, 3.618, etc.) extensions that are typically where major trend reversal occur.
1.618 is to fibs extensions what 0.618 is to fibs retracements: a high probability , that becomes even more probable in an index type instrument than in a sub sector. People who are practiced in analysis are well aware of this statistic of the stock market.
This is, of course, a hindsight call, regarding the 1.618 . However, many other technical and fundamental factors have me convinced that this is only the start of a slide that will last a while longer, perhaps years.
The momentum is on the side of the bears now, after the slow building "rounded top" breakout with some major now sitting above the price ( that in some cases have remained under the price for years) and many of the bigger moving averages starting to roll over.
We have the wild swings (price ) that's typical of the starts to bear markets. Once a bit more uncertainty (fear) settles in for the general market as price repeatedly stalls at above resistances, we will see some violent price slides, IMO .