You are correct.
The Manual option is no longer visible. You can still achieve the same result by doing a quick open using Cmd+Shift+O then type the name of any file you want to open. Press the Alt key and the file will open in the Assistant Editor Window.
As others have noted the new add editor button allows you to add new editors and open files into them. You could file a bug report but you may want to consider that you're likely to get a "works as designed" type of reply.
You can always change or add new keyboard shortcuts to get the behavior you want by viewing the preferences key bindings area and search for the command you want to shortcut.
Note
You can still ctrl+drag from storyboard to the class by the Adjust
Editor button in the upper right of the storyboard. Then just select
Assistant Editor on the list. After which you can make IBOutlets/IBActions.