Scott Ostler: If Kaepernick doesn’t get a shot this year, he never will.

The 2021 NFL season might be Colin Kaepernick’s last chance.

Between now and the end of the season, probably about 15 recycled, substandard quarterbacks will be dusted off and signed as backups by desperate NFL teams.

None of those teams will be desperate enough to call Kaepernick, though, and time is running out.

Hmm. Kaepernick is 33 and healthy. Alex Smith was given the opportunity to make a comeback at age 36 after spending nearly two years rehabbing a destroyed leg.

Kaepernick’s chances of getting that call grow dimmer by the day. And yet, at the beginning of his fifth season in exile, the one-time 49ers Super Bowl quarterback looms over the NFL and over America.

Every social injustice protest in the NFL, in sports, or in society, has a link to Kaepernick and his 2016 kneel-downs. Every signing of a backup quarterback revives the Kaepernick specter. Now there’s a book coming out about the impact Kaepernick had — still has — on athletes and on our culture.

The NFL’s ruling powers must be singing the old country song, “How Can I Miss You if You Won’t Go Away?”

The new book is “The Kaepernick Effect: Taking a Knee, Changing the World,” by Dave Zirin. Sports editor of the Nation, Zirin is a noted commentator who has written nine other books on the tangled web at the intersection of sports, politics and social justice.

For this book, Zirin interviewed people, mostly at the high school and college level, who were influenced and inspired by Kaepernick’s 2016 protests. Powerful stories about powerless people who discovered empowerment.

Jaelun Parkerson, a youth football player in Beaumont, Texas, took a knee, which led to his coach being fired, his team being disbanded, and eventually to the formation of a five-team league that started with funding from NFL players.

“There are people who say taking a knee is passe, or doesn’t mean anything,” Zirin said in a phone conversation. “Well, if you’re in Beaumont, Texas, you’d better believe it means something. If you’re going to take a knee during the anthem, you know you’re basically going to be walking through hell in a gasoline suit for your troubles.”

The stories have some depressing common features. Heavy-handed attempts by authority figures to squash the protests ... racist taunts, bullying, violence ... death threats, of course, the calling card of the psycho fringe.

Here’s another common theme: The subjects in Zirin’s book all encountered hate far more intense than what they anticipated, they endured abuse and intimidation, they saw the dark side of the humanity, yet of them regretted his or her decision to protest.

The many stories form a big picture of Kaepernick’s impact.

“I don’t think he did it consciously, but Kaepernick offered them a language, and a method of struggle that they could take to their communities,” Zirin said. “He provided them with a key they could use to open the door. Before him, they had no road map.”

Kaepernick’s road map guides on.

Zirin, who wrote the book independent of Kaepernick, clearly admires the man for his resolve and courage. Zirin once wrote a book with John Carlos, the bronze-medal sprinter who raised a gloved fist on the Olympic medal stand in Mexico City with gold medalist Tommie Smith. Carlos told Zirin in 2016, “I did what I did in 1968 and that’s something I’ve had to reckon with for 50 years. I did just that one moment. This guy Kaepernick’s doing it for four months.”

And is still doing it, although out of the spotlight. One of Zirin’s interviewees said she was stirred to action while attending a Kaepernick “Know Your Rights” camp.

Zirin finds it ironic that some people who accused Kaepernick of seeking personal attention with his protests now criticize him for his silence and lack of visibility. Also ironic: Whenever a protest turns violent, some people say, “Why can’t they protest

Kaepernick is still an effective crusader, or disturber, in his role as blackballed QB. I believe he would have even greater positive impact were he to get another shot at the NFL. Which is one reason he won’t. Team owners and coaches fear a Kaepernick return would raise protests from a segment of fans rattling their foam sabers, threatening to take their cash elsewhere. Cornhole, anyone?

Some team owners also fear the wrath of a certain political tyrant, still very active, who has warned the owners about playing ball with Kaepernick and his ilk.

It would take courage for a team to sign Kaepernick, and Branch Rickey is dead.

So Kaepernick won’t be suiting up. Not this season, his last realistic chance. But he’ll be around.