For UIDropDown, the new stuff is related to Custom Frame. Comments from UIDropDownMenuTemplates.lua:
Lua Code:
-- Custom dropdown buttons are instantiated by some external system.
-- When calling L_UIDropDownMenu_AddButton that system sets info.customFrame to the instance of the frame it wants to place on the menu.
-- The dropdown menu creates its button for the entry as it normally would, but hides all elements. The custom frame is then anchored
-- to that button and assumes responsibility for all relevant dropdown menu operations.
-- The hidden button will request a size that it should become from the custom frame.
Regarding to the possible workaround mentioned by Ellypse, I think technically it may work. But this would also introduce some problem for other addon's developer when then need to use those UI element.