A database in Plesk is marked as Used by an installed web app and cannot be marked for deletion;
The option Remove Database is grayed out at Plesk > Domains > wordpress.tld > Databases
Databases marked as Used by an installed web app are created automatically with a web application (such as WordPress, Joomla, Drupal, etc.).
It is not possible to delete these databases separately from a web application.
The only possible way is to remove a whole web application:
Remove the application in Subscriptions > example.com > Applications.