The specific human right is usually referred to as "freedom of association". You can read more here: https://en.wikipedia.org/wiki/Freedom_of_association
You need something more than freedom of association to justify unions, rooted in the recognition of bargaining disparities between employers and employees.
That's something the libertarian left and libertarian right surely agree on - minimising the application of the monopoly of force and all that.
So you assume both employers and employees can freely organize and associate. That's your starting point.
Now - we've decided that the right to join a union should be protected (it evens out an existing power imbalance) and that collusion to force down wages should be illegal (because it amplifies an existing power imbalance to the detriment of society at large)
We're moving away from areas where I'm confident I know what I'm talking about, but I think as a society we've decided that while anti-competitive laws do infringe on the rights of business leaders, we're trying to balance their rights with those of everyone else, and the laws are necessary to prevent a permanent class divide between business leaders who cannot be challenged, and workers under them. In the long run, allowing complete free association among CEOs would limit the freedoms of the rest of society.
Society is a constant project of balancing various conflicting rights, and this is one of many cases where we limit the rights of a few to defend the rights of many.
Fast forward many decades later. This is a right you and I both enjoy. (This is also why we both have time to shitpost on HN.) If there was a serious proposal to take it away, you and I would probably be standing side by side on the street demanding our rights back.
It’s easy to talk a big game about human rights when your rights aren’t on the line.
By looking into the declaration of human rights: https://www.un.org/en/universal-declaration-human-rights/
FWIW, You can pick out any other number of declarations of human rights and the right to form and join trade unions is in them. It's very widely considered to be a human right, not some fringe thing.
Obviously not. Real-world cases in point:
* Firing people for being queer
* Firing people for objecting to sexual harassment
* Firing people for going to the bathroom
"Life, Liberty and the pursuit of Happiness"
This is, legally speaking, a meaningless document.