Implementing delegate methods in an UIView class
I personally separate my code in my view controllers using the comment //MARK: - which adds shortcuts to your code in the top navigation bar (the one that shows what file you're currently at). I would keep @IB related properties in your original view controller rather than make a extension for the @IBActions.

Also, making an extension for delegates is good practice because it helps separate your code from Apple's delegate methods. I don't see anything wrong with this approach since you can either make an extension to your view, or add it within the original class declaration, but as far as MVC goes, this would be the ideal approach.

As long as you're not doing heavy data/logic/algorithms going on, then you're good. 

as for the delegate.doSomething() call, I don't see the point of using delegate since the scroll view delegate code is in an extension of the view, which means doSomething() is already accessible.