6 files declare their use of ViewUI in Drupal 9

ViewsUIController.php in core/modules/views_ui/src/Controller/ViewsUIController.php

views_test_classy_subtheme.theme in core/modules/views_ui/tests/themes/views_test_classy_subtheme/views_test_classy_subtheme.theme

Implements Views UI alter hooks in a theme to provide test coverage.

ViewUIConverter.php in core/modules/views_ui/src/ParamConverter/ViewUIConverter.php

ViewUIObjectTest.php in core/modules/views_ui/tests/src/Unit/ViewUIObjectTest.php

WizardPluginBase.php in core/modules/views/src/Plugin/views/wizard/WizardPluginBase.php

WizardPluginBaseKernelTest.php in core/modules/views/tests/src/Kernel/Wizard/WizardPluginBaseKernelTest.php