New Menu Option

In the Pages Menu you can now select "New menu section".  This allows you to create sub pages under the parent path without having a parent page.
See the 'Sub Pages' navigation item in this site.

Nest pages inside a dropdown menu

1. On your computer, open a site in new Google Sites.

2. At the right, click Pages.

3. Click the page with the menu you want to update.

4. At the bottom right, hover over Add .

5. Click New menu section .

6. Add a menu section name.

7. Click Done.

8. You can drag and drop pages into your new menu.

Tip: When you preview your site, you can see your nested pages by hovering over the new menu.

From https://support.google.com/sites/answer/98216?hl=en#zippy=%2Cnest-pages-inside-a-dropdown-menu