Is there one? This seems so useful for vision impaired users. When a UIView has the adjustable UIAccessibilityTrait, the entire view can be focused as one unit, and the user can increase/decrease the amount of that unit or continue to swipe to other focusable elements on the screen.
As an example, imagine a screen containing a list of toppings to add to ice cream. Each topping has a plus (+) and minus button (-). With the adjustable trait, the user can swipe through the toppings, and then stop and swipe up/down to add/remove the one they want. Without the adjustable trait, each topping needs up to 3 elements on the screen that each take focus: Sprinkles (label), add button, remove button. Swiping through all these elements sounds like a nightmare for a vision impaired user.