Rachael McDaniel at FanGraphs notices the problems with fly balls this season:

I can’t confirm that this is true — sadly, Statcast doesn’t track “balls lost in the light.” But so far this season, it’s seemed to me like more catchable fly balls have disappeared into the sky than is usual for a 30-day period of major league baseball. Surveying the range of fly balls and popups with an xBA of less than .100 that didn’t result in outs, the earliest lost ball I can find is from July 27, when the Blue Jays’ Derek Fisher sent a fly ball into the right-field sunshine. Adam Eaton thought he could see it. Victor Robles, from his vantage point in center, knew that he couldn’t. And as Robles desperately sprinted over, Eaton tried even more desperately to correct his positioning. He leaped backward, the ball spinning away off his glove. His recovery attempt had only made things worse.

FanGraphs.com

My belief is that fans following the ball in flight is more important than previously realized. The Hunter Pence example in the article is informative in this regard. Pence has no clue the entire time as to the location of the ball. If there were fans in the stands, and all the fans were looking in a direction that suggested the ball was behind Pence, he might have started back and he might have picked up the ball. The fans would serve as a massive radar array.

If the data were available, one might look at the frequency of missed easy plays in sparsely attended minor league games versus well attended minor league games. If there were a critical mass of fans needed to cue outfielders, and that number was low, it could give MLB clubs a lower limit on how many fans could be allowed into a ballpark once they are allowed to return.