17 files declare their use of RfcLogLevel in Drupal 9

AreaDisplayLinkTest.php in core/modules/views/tests/src/Kernel/Handler/AreaDisplayLinkTest.php

BigPipeTest.php in core/modules/big_pipe/tests/src/Functional/BigPipeTest.php

bootstrap.inc in core/includes/bootstrap.inc

Functions that need to be loaded on every Drupal request.

dblog.admin.inc in core/modules/dblog/dblog.admin.inc

Administrative page callbacks for the Database Logging module.

DbLogController.php in core/modules/dblog/src/Controller/DbLogController.php

DbLogTest.php in core/modules/dblog/tests/src/Functional/DbLogTest.php

EntityReferenceSettingsTest.php in core/modules/field/tests/src/Kernel/EntityReference/EntityReferenceSettingsTest.php

errors.inc in core/includes/errors.inc

Functions for error handling.

FakeLogEntries.php in core/modules/dblog/tests/src/Functional/FakeLogEntries.php

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

MigrateMessage.php in core/modules/migrate/src/MigrateMessage.php

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

OEmbedSourceTest.php in core/modules/media/tests/src/Kernel/OEmbedSourceTest.php

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

responsive_image.module in core/modules/responsive_image/responsive_image.module

Responsive image display formatter for image fields.

SecurityAdvisoriesFetcherTest.php in core/modules/system/tests/src/Kernel/SecurityAdvisories/SecurityAdvisoriesFetcherTest.php

ViewsIntegrationTest.php in core/modules/dblog/tests/src/Kernel/Views/ViewsIntegrationTest.php