It was removed in https://jira.ez.no/browse/EZP-31425. Support Team confirmed it was used by some clients so it might be a good idea to bring it back sometime in the future. It is not compatible with Symfony 4/5 due to a few issues:

• extends removed ContainerAwareCommand
• uses Container:get
• relies on deprecated handlers a lot

ref. https://github.com/ezsystems/ezpublish-kernel/blob/7.5/eZ/Bundle/EzPublishMigrationBundle/Command/LegacyStorage/CleanUpRelationTypeEqZeroCommand.php