What is a security.txt file?
A security.txt file is a way for websites to show how security researchers can get in contact and whether there is any bug bountry or vulnerability disclosure policy.
gotsecuritytxt.com scans the top 250 sites daily (you are viewing top sites in Great Britain currently) for their security.txt file and parses it.
You can also query any website to see if there is a security.txt file and whether it appears to be formatted correctly.
You can find more about the security.txt file and the RFC status on securitytxt.org