Historically, buying single digit extreme fear has ended very well if you are patient long enough. however, most people get this wrong.

The fear and greed index measures the crowd emotion and doesn't distinctly determine a bottom. A lot of people just blindly ape into the market any time this indicator flashes extreme fear.
That only works if you sell during extreme greed in a bull market. It also only works if you have confluence of said indicator with price action (momentum, moving averages, price structure, time etc). In confluence with some of this analysis, I was happy to press buy on $BTC and a few altcoins last week when we traded into single figure extreme fear.
I still think price could trade lower and I've got capital waiting if that happens, but a smart trader / investor prepares for all scenarios and I found decent value last week.