According to ProFootballReference, 2018's regular-season attendance totalled 17,177,581 attendees, down from 2017's 17,253,425, and is the lowest since 2010's 17,140,859.
The past 10 seasons are as follows. Franchise relocations and some other events are noted. For sake of simplicity, events including, but not limited to the following are not listed: Team records in any given season, Player suspensions, ticket prices or special discounts.
2016: 17,788,671 - Colin Kaepernick creates kneeling controversy starts during preseason. The Rams play their first season in Los Angeles since 1994.
2017: 17,253,425 - In September, Donald Trump explicitly condemns the National anthem protests. The Chargers play their first season in Los Angeles.
For the curious, the last time attendance dipped below 17 million was all the way back in the 2002 season, where it was at 16,979,371. And it dipped below 16 million, and even 15 million, back in 1998, where it was at 14,960,910. The lowest attendance recorded on the site was in 1992, the first year attendance was tracked, where it totalled 13,135,413.
To sum everything up, there's been a definite decline in attendance since kneeling became a thing, making the first dip below 17M in over a decade and a half a very real possibility, but odds are it won't go below 16 million any time soon, barring unforeseen circumstances.