Equivalent of dynamic type "automatically adjusts font" setting for UIButton in Interface Builder?

Apparently there isn't, but it's not very hard to fix. You can make an extension on UIButton with an @IBInspectable property:

extension UIButton {

    @IBInspectable
    var adjustsFontForContentSizeCategory: Bool {
        set {
            self.titleLabel?.adjustsFontForContentSizeCategory = newValue
        }
        get {
            return self.titleLabel?.adjustsFontForContentSizeCategory ?? false
        }
    }
}

Now you can simply turn it on for every UIButton (or any of its subclasses).