But there wasn’t a major economic impact. From a new paper in the Journal of Sports Economics, by Judah Brown and Brandon Sheridan. The full abstract:

The National Football League’s (NFL) television ratings decreased by approximately 8% during the 2016 season, then a further 10% the following season. These declines coincided with league-wide national anthem protests initiated by Colin Kaepernick at the beginning of the 2016 season. Existing research identifies many determinants of demand for sporting events, but athletes’ protests are seldom considered. We use detailed data on players’ protests and television ratings to construct a new, game-level panel for the four NFL seasons between 2014 and 2017. Our results show protests are statistically significantly associated with lower TV ratings, but the economic magnitude is relatively muted.

Article here, but you might need institutional access. It’s also worth noting that viewership declines might be overstated because we don’t really have great data on illegal streaming, an increasingly popular option, especially for younger viewers. Furthermore, other research suggests that while there are fewer people watching football, those who do still watch are consuming more than ever.