It's interesting (in my mind, disappointing) that Github lists only active staff-- in my mind, humans.txt should document all the people who have ever worked to bring a website to its current state.
It was my understanding that it was supposed to – kind of like the credits at the end of a movie. It's not a list of people who work with the production company when you see the movie, just when it was made. Add people whenever you want, but taking people out just makes it another, less readable "About Us" page.
I wondered about that. The spec isn't clear about it, so we've been taking people out if they leave.
The old company that I worked for added a "Thanks" section which I am quite happy about: http://mmm.dk/humans.txt
It's not listing all active staff. I wonder what the requirements are.
If I had to guess, I'd say it lists the people who take the initiative/care enough to edit the file for themselves and commit their updates. Github is famous for having an open organization, so it wouldn't surprise me if there were no designated maintainer/required process to add yourself to humans.txt.
It may just be a script that gets all the active org members who can edit github/github and copies their profile info.