There is an apiResponse field in View.php. Some of the contents come from renderApi (shared by the actual API).
However, some are put directly there, which means the data is only available in the template if it's a server rendering. That in turn means the template output differs.
This is a major problem (the architecture is built on the assumption that individual templates behave exactly the same on client and server), and causes bugs like T74655: Shows 'Post a new message to ""' after changing sort order.