Donald Trump was among his critics, describing the protest as unpatriotic and suggesting players who refused to stand should lose their jobs.
Kaepernick left the 49ers in March 2017 after opting out of his contract. The franchise subsequently said he would have been released anyway.
Despite previously guiding San Francisco to the Super Bowl and throwing 16 touchdowns in his final season, he never secured another team.
He later alleged that NFL owners had colluded to keep him out of the league because of his activism, with the dispute ending in a financial settlement in 2019.
