It’s biaised to show this without the dates at which features were introduced. A lot of the downtimes in the breakdown are GitHub Actions, which launched in August 2019; so yeah what a surprise there was no Actions downtime before because Actions didn’t exist.
Check the breakdown page. Like yes the magnitude is reduced obviously for individual services. But they all show the same trend.
Even worse, those features show "100% uptime" pre-existence on the breakdowns page too.
You can click on "Breakdown" and then on "Actions" to hide it.