Drupal 8 hook_uninstall not implemented
Uninstalling civi using the UI or drush doesn't currently uninstall anything it just tells drupal basically to uncheck the box on the modules list page, but all the data is still there. This leaves drupal+civi in an inconsistent state and trying to check the box again leads to a borked UI because civi keeps trying to do a first run but can't because as far as it's concerned it's already installed.
In drupal 7 hook_uninstall is implemented and drops all the civi tables.
https://github.com/civicrm/civicrm-drupal-8/pull/45#issuecomment-698982257
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