Delete a site or your account
Export first. Deleting a site removes everything for it at once; deleting the account pauses every site and purges it after seven days.
Deleting a site (settings page, type the domain) removes every row for that site and the site itself at once. The domain is free to register again.
Deleting the account (account page, type your email) pauses every site and schedules the purge after seven days. Until then, Keep my account cancels it.
After the purge, every site, every row, your preferences and your login are gone. The plugin on your WordPress site keeps its own undo table until you deactivate it there.