The page must work even without frontmatter specified, the slug to build the url is extracted from the filename and page title is extracted from the first line of the markdown.

By default this page doesn't show in menu, unless explicitly added to the menu: in the configuration file marmite.yaml

menu:
  - ["My Page", "no-metadata.html"]

If the menu is not specified, than marmite will add tags, archive and pages listing to the menu.

Please consider giving a ☆ on Marmite Github repository, that helps a lot!