If you set SYMFONY_DEPRECATIONS_HELPER to weak_vendors and then have a test uses the @runTestsInSeparateProcesses annotation and does not have an @group legacy even though deprecations are triggered from non-vendor code, they are ignored. This is because the with-process-isolation-silenced-error is actually triggered from \Symfony\Bridge\PhpUnit\Legacy\SymfonyTestsListenerTrait::endTest() which is vendor code for most projects (eg. Drupal). For a real world example see: https://www.drupal.org/project/drupal/issues/2934336