If you are on Middle East(yes!)/North American language feature set for InDesign then the New Document which, as of now, is not available for your feature set. The preference that you have mentioned ("Use Legacy New Document Dialog") is to disable that "New dialog" and hence it is not visible to you.
Illustrator and Photoshop has provided the new dialog for Middle East but InDesign has yet to implement this feature for Middle east feature sets. As for now, you can try out the templates which are new in the "New Document Dialog" can be tried directly from Adobe Stock website. You can manage the InDesign templates in CC Libraries panel by selecting Stock Templates library.
In that same review, David Blatner also noticed that this 'better, improved' dialog got displayed glacially slow. Will that part of this new feature be faithfully copied to the ME version as well? And will it come with the same work-around "use legacy dialog" in the Preferences?
(Perhaps the option to switch back should be mentioned in InDesign CC new features | New modern and customizable UI under the heading "Start projects faster". As it is, that text reads kind of ironic.)
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Yes, we have been hearing a lot of such performance issues with the new dialog.
Like always, based on the feedback's and performance reports an appropriate action to cater to the "slowness" may be taken.