Started by humanstxt.org, the idea was to put more of a focus on people rather than machines. A humans.txt file is a text file a website owner can put in the site root file, the same place you would put your robots.txt file. It gives users more information on the people behind the website. Users can access it by typing /humans.txt behind a domain name.
The team behind it describes the initiative as helping with “knowing the people behind a website”. You can mention whoever you feel contributed to the website, be that the developer, designer, website owner or marketing team.
Is it mandatory?
No. You don’t have to have one and you won’t be punished for not having one. It’s just something you can do if you have a little extra time, or if you want to properly credit individual team members who helped with the site.