Github was originally written in Ruby on Rails back in 2008. React was released to the public in 2013 (though it was in use at Facebook prior to that.)

If we inspect the source for Github back in 2010 on the Wayback Machine[0], we can see that they were using jQuery like most sites of that time period.

[0] https://web.archive.org/web/20100115102310/http://github.com... (interestingly, elan/plex shows up as a recently updated repo!)