I don't remember that happening so much (if ever) in, say, 2016. But the frequency of noticeable incidents seemingly has been rising steadily since around 2023. The Azure migration apparently only exacerbated it.
I remember seeing unicorn daily and "webhook delivery delayed" weekly. I think it got better, but also they got more traffic, now millions of agents read files separately over and over again.
I remember it going down semi-regularly in the 2013+ era, and seeing HN posts about it. Especially if you were using a package manager reliant on GitHub like Cocoapods. It seems to me it is more "impactful" on the dev community now that they have gone past just being a centralized Git server for the team, to being the thing that does deploys and all sorts of other things.
It was not nearly as bad... I remember our company migrating to github.com, and believe it or not, it was significant performance/uptime benefit over our self-hosted instance.
(And the first thing to go was occasional 500's on github-hosted files.. the core service itself - git, PR, actions - were pretty stable until recently)