They have some self-reported data [1]. You could scrape that and transform it. Eyeball analysis suggests it started in december 2019-february 2020, and rapidly

They have some self-reported data [1]. You could scrape that and transform it. Eyeball analysis suggests it started in december 2019-february 2020, and rapidly went downhill starting april. [1] https://www.githubstatus.com/uptime?page=2 https://www.githubstatus.com/uptime?page=2