Over the last 90 days the status page (https://www.githubstatus.com/) shows around 2 nines of uptime for most services.

The Missing GitHub Status Page (https://mrshu.github.io/github-statuses/) gives a better overview of their frequent outages.

"We need an SCM with five nines of availability. What about GitHub?"

"Well. It's got a nine in it"

"What percentage??"

"Nine"

at least enterprise is better https://us.githubstatus.com/posts/dashboard

I trust that about as much as I trust Volkswagen's emissions numbers.