GitHub Actions, on the other hand, is frequently used in the same workflows as the base GitHub product, so it's worth considering both separately and together, much like various Azure services, whereas I see no reason at all to consider an aggregate "Microsoft" downtime metric that includes GitHub, Azure, Office 365, Xbox Live, etc.
The most useful, metric, actually, is "downtimes for the various collections of GitHub services I regularly use together", but that would obviously require effort to collect the data myself.