Stoically, it is one of the few things that certainly cannot be changed. But its impact does change people. If reality includes the entire timeline, then it does exist, but if it is just the present moment, then it has existed - but not anymore.
There is an absolutely gigantic - possibly infinite - spectrum of possible versions of reality. However, since only one timeline will exist, you can think of "parallel instances" being killed in extremely rapid succession.
It can also be said that in cause-and-effect, the exact smallest unit of time, as per the laws of physics, will determine the "state" of the exact smallest next moment in time - so there might always have been one timeline all along. Though the amount of causes that can effect the effectis incomprehensibly large!!