It seems there are times when we want to know when a timeline is at a particular stage so we can do a few things.
My original thoughts were in the scope of questions like these.
The main thing is we have to poll or request the currenttime in a time line manually or write our own code using mutation observer, poll via setInteval or/and requestanimationFrame to run code when the time is right.
The closest thing to a timeline onupdate is using a Math Equation within an easing to run a hype function ( outside its own scope) aliased to a global. ( also which cannot be accessed within a sprite )
An on event or as @MaxZieb suggests an optional registration to a HypeTimelineUpdate event would possibly make it easier to innovate and understand and writ for the masses. In particular doing away with the cumbersome setInteval and complex mutation observer.
This is not saying it will not have it's own challanges...