Resetting or deleting content
Shifter doesn't offer to reset WordPress, including content, media, and other website resources, and other website resources.
Deleting the website's current content
Alternative ways
Delete the site and create a new one
Deleting the site and creating a new one would be a good alternative.
For more details: Creating a new site on Shifter
Use reset plugin
Some plugin offers to delete all website content, media and other resources.