There is the GitHub Status Page [0], which doesn't display aggregate stats, but you could scrape it and do the analysis.

I suspect what you're getting at is that the downtime might evaluate to multiple days over the course of the year. Maybe that's true, idk. I'd be curious, but you'd probably want to do the analysis separately for different services (e.g. Actions vs. Package Registry vs. Git outages all have different effects on build infrastructure downtime).

[0] https://www.githubstatus.com/history