Also, any non-trivial system has failure modes that are not binary. Like, if your convertible car has the roof stuck down you can drive it, but you will be sad if it rains. So, is the car “up” or not? Similarly, if your site has writes offline for a database upgrade, but reads work fine, is it “up”?

The concept is just advertising and something that makes sense to senior management, but provides no actual useful metrics.