Why are AndroidX classes in beta or alpha phase final?

Question

I like to extend the new ViewPager2 from the AndroidX components. Found such a nice example how to set ViewPager2 fragments with dynamic height (SO answer here)

But as far as I can see class files in beta, maybe alpha phase are all marked final. Impossible to extend:

public final class ViewPager2 extends ViewGroup {

Is it common to mark classes final during development and when 1.0.0 is out they remove final?

dependencies {
    implementation 'androidx.viewpager2:viewpager2:1.0.0-beta03'
}