OK. Since trying to contact a human at Microsoft is an exercise in madness, for questions of this nature it's usually best to go to the Microsoft help forum. There are one or more threads about this very problem: https://answers.microsoft.com/en-us/outlook_com/forum/oemail-osend/has-view-email-in-source-form-been-removed-from/e0b6b7f6-913e-4025-8e0d-cdd889b3c854
Here is what one of the mods had to say as of 9/23/2018 3:23:47 PM (EDT, I'm guessing):
You are correct there is no way to view headers in the New version.
Please add request to Uservoice they are constantly updating missing services in the new version and I agree this is an important feature. https://outlook.uservoice.com/forums/284136-out...
We are only moderators on this site not Microsoft employee's.
Sorry the feature is not available I realize this is not the response that you'd like to hear.
So, Uservoice has the following thread on this subject. It's a month old and addresses the beta version of the layout. If this newest update is the non-beta version, there may be a more relevent thread.