I actually couldn't tell you the exact version that was installed on this site. I do know that due to the configuration still being in the filters... "Migrate Drupal" (a core module) cannot migrate Filters from Drupal 7 to Drupal 8 after this module has been enabled on Drupal 7. It fails with the error:

The "pathologic" plugin does not exist.                                                                                       [error]
(/var/www/drupal/build/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php:52)

There wasn't a reason placed in the D6 version as to why this issue wouldn't be fixed... see #940960: Database entries not cleaned up during uninstall.

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generalredneck created an issue.