It's true that they reduced stadium capacity in 2015 and 2017. At its peak, Memorial Stadium's capacity was 87,147 and is now 85,458. That's a whopping reduction of nearly 1700 seats. Btw, 2019 average attendance was still more than 89,000 (still above seating capacity).
As the linked articles point out, many of Nebraska's long time season ticket holders are "up in age" and it is questionable as to whether they will attend games this Fall. Streak is at 375, but that may come to an end when the whistle blows for the season opener.

https://www.cornnation.com/2020/5/1/21241191/friday-flakes-nebraska-football-sellout-streak-likely-will-not-survive-the-virus

http://www.dailynebraskan.com/sports/curious-cornhuskers-analyzing-nebraska-football-game-attendance-from-last-5-years/article_5f7569ba-5db3-11ea-8bb2-53ddb2a7cdc6.html