Copy Site - new Google Sites
Use a new Google Site as a template and copy a site using the duplicate site feature (or create a backup) in new Google Sites
Duplicate a new Google Site
Edit the new Google Site you want to copy
Use the More (three-dot) menu, on the left of the publish button, and choose Duplicate site
Enter the File name you want for the copied site in Google Drive, the Google Drive Folder you want the copied Google Site to appear in and choose if you want to Share with the same editors
Once the copy process begins you will see one of two options:
if your site is small (50 pages or less) then you will see the link for the new site appear in the notification bar at the bottom of the site editor
if your site is large (more than 50 pages) the new Google Sites editor will notify you to check your email for site copy updates
Notes | Copying a new Google Site
You can copy small sites in Google Drive but large new Google Sites will fail to copy and you need to open the site in the editor and copy from there.
Only editors and the owner can copy the site.
At the moment you cannot prevent editors from copying a site.
Availability | Large site copying | new Google Sites
It will take 15 days for full rollout of the feature to rapid release Google Workspace Domains (released on 17 April 2019) and scheduled release will start the 15-day roll-out at the same time.