I think a dropdown API would be dangerous to add, as it assumes that Vector/Vector 2022 will always render menus in dropdowns and it's possible at some future date, we may change the API to have different behaviour. The addPortlet and addPortletLink methods have always been about creating the generic structure rather than visual appearance. Having a method called addDropdown also creates the concern that for skins where the concept of a dropdown UI doesn't exist, the function would lead to some confusion as the resulting element would not be a dropdown.

I think we are lacking necessary foundations to even be able to consider such an API at this time, in particular we need:

1. A dropdown component in Codex that renders a dropdown identical to Vector
2. Use of that new dropdown component inside Vector
3. A new API that replaces addPortletLink and addPortlet for creating a Codex dropdown component (something best suited for Design-System-Team).

At the current time, I don't think gadget developers plugging into a skin can be responsible for UI in a way that we can stable - for now the responsibility needs to lie with the skin to do what makes sense with hints from the gadget developer. The current API at least permits a way to do this, but in a way that doesn't promise "this will always be rendered as dropdown forever.

I've opened T365661 to advertise this API, collect some more feedback, for more discussion.
If someone feels strongly this is needed, I recommend raising a new ticket for getting the dropdown component added, and then if and when that's done we can circle back to doing something inside skins.