As per Snopes...
NFL players were not required to be on the field during the playing of the U.S. national anthem prior to 2009.
WHAT'S FALSE
Players always had the option of standing on the sidelines during the national anthem, and there is no evidence that "paid patriotism" initiatives begun in 2009 required them to do so.
Prior to 2009, whether or not to appear on the field for the anthem was left up to teams discretion. In one instance, the Philadelphia Eagles and Washington Redskins did so on 24 November 1963, just two days after the assassination of President John F. Kennedy.
As per CNN...
[T]he connection between paid patriotism and players being mandated to be present for the anthem is tenuous. The report does mention several instances where teams were paid for anthem performances, but that was about the specific artist or presentation. There is nothing in the report to suggest teams were paid or coerced into pulling players on to the field as part of paid patriotism initiatives.
In fact, Pentagon spokesman Army Major Dave Eastburn recently clarified the Defense Departments current relationship with professional sports.
DoD does not require or request that athletes be on the field during the playing of the national anthem when military members are part of the patriotic opener, he said in a statement to CNN.