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