An aggregate number like that doesn’t seem to be a reasonable measure. Should OpenAI models being unavailable in CoPilot because OpenAI has an outage be considered GitHub “downtime”?
I think reasonable people can disagree on this.
From the point of view of an individual developer, it may be "fraction of tasks affected by downtime" - which would lie between the average and the aggregate, as many tasks use multiple (but not all) features.
But if you take the point of view of a customer, it might not matter as much 'which' part is broken. To use a bad analogy, if my car is in the shop 10% of the time, it's not much comfort if each individual component is only broken 0.1% of the time.
As long as they brand it as a part of GitHub by calling it "GitHub Copilot" and integrate it into the GitHub UI, I think it's fair game.