The quarterback Colin Kaepernick ignited impassioned debates over race, activism and free expression after protesting police shootings of black men by kneeling during “The Star-Spangled Banner” at National Football League games in 2016.

The president tweeted his anger at him and dozens of other players who also knelt in protest during the national anthem. Some fans boycotted games over the players’ actions, and some boycotted the N.F.L. with the belief that Mr. Kaepernick, who failed to land a job the next season, was being blacklisted for his leadership in the movement.

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“That is often difficult to do because parties typically don’t leave a written record of their illegal maneuvering,” said William Gould, who was chairman of the National Labor Relations Board at the time of the Major League Baseball strike in 1994.