17 files declare their use of FileStorage in Drupal 9

CachedStorageTest.php in core/tests/Drupal/KernelTests/Core/Config/Storage/CachedStorageTest.php

ConfigAfterInstallerTestBase.php in core/tests/Drupal/FunctionalTests/Installer/ConfigAfterInstallerTestBase.php

ConfigFileContentTest.php in core/tests/Drupal/KernelTests/Core/Config/ConfigFileContentTest.php

ConfigInstallProfileOverrideTest.php in core/modules/config/tests/src/Functional/ConfigInstallProfileOverrideTest.php

ConfigSchemaTest.php in core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php

ConfigStorageStagingTest.php in core/tests/Drupal/KernelTests/Core/Config/ConfigStorageStagingTest.php

DefaultConfigTest.php in core/tests/Drupal/KernelTests/Config/DefaultConfigTest.php

DemoUmamiProfileTest.php in core/profiles/demo_umami/tests/src/Functional/DemoUmamiProfileTest.php

FileStorageFactoryTest.php in core/tests/Drupal/KernelTests/Core/Config/FileStorageFactoryTest.php

FileStorageTest.php in core/tests/Drupal/KernelTests/Core/Config/Storage/FileStorageTest.php

FunctionalTestSetupTrait.php in core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php

install.core.inc in core/includes/install.core.inc

API functions for installing Drupal.

MediaDisplayTest.php in core/modules/media/tests/src/FunctionalJavascript/MediaDisplayTest.php

ModuleTestBase.php in core/modules/system/tests/src/Functional/Module/ModuleTestBase.php

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

ViewsConfigUpdaterTest.php in core/modules/views/tests/src/Kernel/ViewsConfigUpdaterTest.php

ViewTestData.php in core/modules/views/src/Tests/ViewTestData.php