The documentation here states that the saveItem command group placement contains the Save As as a default command, but it doesn't appear.
I have my document type specifying multiple 'writableContentTypes' - I expected this would enable the save as.
How do I do this?
On Mac you have to hold down the Option key to make the Save As menu item appear in the File menu.
The Save As menu item is meant for saving the current document with a different file name. The menu item doesn't have anything to do with multiple content types. If you have multiple writable content types, the Save panel has a menu to choose the file type to use to save the document when you choose File > Save. You don't need the Save As menu item to specify a file type for a saved document.