It feels like the command has been removed from the object model in release 1903. Some users are not experiencing this though. Read through entire thread.
The vba command
application.Dialogs(wdDialogFileNew).Show
will also crash Word.
I have long been an advocate of using this dialog.
File New Variations in the Versions of Word
The people who can make changes to Word seldom, if ever, read anything that is posted in this help forum.
Like you, I am upset about this. Here is my suggestion on UserVoice. Please vote and comment!
.https://word.uservoice.com/forums/304924-word-for-windows-desktop-application/suggestions/37338250-classic-legacy-filenew-dialog-removed-from-1903
Also, use the Feedback mechanism!
I have started a discussion of this problem:
Classic / Legacy FileNew dialog gone! Attempt to
use crashes Word. Release 1903
So far there are four reports I've seen, including mine. I will post a link to any that I see in that discussion by editing the initial post. I would invite further discussion of this to that discussion.