Hi, I am the author of Social Fixer.
The github site does exist, but only if you actually go to the exact url of a file. For example: https://matt-kruse.github.io/socialfixerdata/filters.json
The JSON files retrieved by the extension are configuration data, to adapt to the ever-changing Facebook environment. Hosting them on github allows other contributors to add fixes as necessary, and have them instantly go live to users.