The NFL wanted to have a system in place by the time training camps opened to differentiate within a team’s facility who has been fully vaccinated and who has not, ostensibly to more easily monitor who has to follow which set of protocols. They ultimately left it up to teams to decide how to manage this, with many opting to have unvaccinated players wear, for example, wristbands.
The Pittsburgh Steelers were among such teams, who opted to have unvaccinated players wear yellow wristbands, though as of yesterday, it is not clear if there are any unvaccinated players remaining.