I've just upgraded to KMail 5.4.0 (from 4.something), and I no longer see an option to display the raw source of an email.

Why was this removed? Is there some way to reenable it? This is a feature I really need every once in while...

The option has only been removed from the right-click context menu.
But it's still reachable via the "Message" menu or the keyboard shortcut (should be 'V' by default).
And it's also possible to add it to the toolbar if you want to...

Ah, thanks. I had some trouble finding the option.