33 files declare their use of ConfigFormBase in Drupal 9

AccountSettingsForm.php in core/modules/user/src/AccountSettingsForm.php

AdvancedSettingsForm.php in core/modules/views_ui/src/Form/AdvancedSettingsForm.php

BasicSettingsForm.php in core/modules/views_ui/src/Form/BasicSettingsForm.php

BookSettingsForm.php in core/modules/book/src/Form/BookSettingsForm.php

FileSystemForm.php in core/modules/system/src/Form/FileSystemForm.php

FormTestArgumentsObject.php in core/modules/system/tests/modules/form_test/src/FormTestArgumentsObject.php

FormTestControllerObject.php in core/modules/system/tests/modules/form_test/src/FormTestControllerObject.php

FormTestObject.php in core/modules/system/tests/modules/form_test/src/FormTestObject.php

FormTestServiceObject.php in core/modules/system/tests/modules/form_test/src/FormTestServiceObject.php

ForumSettingsForm.php in core/modules/forum/src/ForumSettingsForm.php

ImageToolkitForm.php in core/modules/system/src/Form/ImageToolkitForm.php

JsonApiSettingsForm.php in core/modules/jsonapi/src/Form/JsonApiSettingsForm.php

LocaleSettingsForm.php in core/modules/locale/src/Form/LocaleSettingsForm.php

LoggingForm.php in core/modules/system/src/Form/LoggingForm.php

MediaSettingsForm.php in core/modules/media/src/Form/MediaSettingsForm.php

NegotiationBrowserForm.php in core/modules/language/src/Form/NegotiationBrowserForm.php

NegotiationConfigureForm.php in core/modules/language/src/Form/NegotiationConfigureForm.php

NegotiationSelectedForm.php in core/modules/language/src/Form/NegotiationSelectedForm.php

NegotiationSessionForm.php in core/modules/language/src/Form/NegotiationSessionForm.php

NegotiationUrlForm.php in core/modules/language/src/Form/NegotiationUrlForm.php

PerformanceForm.php in core/modules/system/src/Form/PerformanceForm.php

RegionalForm.php in core/modules/system/src/Form/RegionalForm.php

RssFeedsForm.php in core/modules/system/src/Form/RssFeedsForm.php

SettingsForm.php in core/modules/media_library/src/Form/SettingsForm.php

SettingsForm.php in core/modules/aggregator/src/Form/SettingsForm.php

SiteConfigureForm.php in core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php

SiteInformationForm.php in core/modules/system/src/Form/SiteInformationForm.php

SiteMaintenanceModeForm.php in core/modules/system/src/Form/SiteMaintenanceModeForm.php

StatisticsSettingsForm.php in core/modules/statistics/src/StatisticsSettingsForm.php

SystemConfigFormTestForm.php in core/modules/system/tests/modules/form_test/src/SystemConfigFormTestForm.php

ThemeAdminForm.php in core/modules/system/src/Form/ThemeAdminForm.php

ThemeSettingsForm.php in core/modules/system/src/Form/ThemeSettingsForm.php

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