6 files declare their use of PrivateKey in Drupal 9

CsrfTokenGenerator.php in core/lib/Drupal/Core/Access/CsrfTokenGenerator.php

IFrameUrlHelper.php in core/modules/media/src/IFrameUrlHelper.php

IFrameUrlHelperTest.php in core/modules/media/tests/src/Unit/IFrameUrlHelperTest.php

PermissionsHashGenerator.php in core/lib/Drupal/Core/Session/PermissionsHashGenerator.php

PrivateKeyTest.php in core/tests/Drupal/Tests/Core/PrivateKeyTest.php

UpdateProcessor.php in core/modules/update/src/UpdateProcessor.php