23 files declare their use of SqlContentEntityStorage in Drupal 8

block_content.post_update.php in core/modules/block_content/block_content.post_update.php

Post update functions for Custom Block.

CommentStorage.php in core/modules/comment/src/CommentStorage.php

contact_storage_test.install in core/modules/contact/tests/modules/contact_storage_test/contact_storage_test.install

Contains install and update hooks.

ContentTranslationTestBase.php in core/modules/content_translation/src/Tests/ContentTranslationTestBase.php

ContentTranslationTestBase.php in core/modules/content_translation/tests/src/Functional/ContentTranslationTestBase.php

EntityTestNoLoadStorage.php in core/modules/system/tests/modules/entity_test/src/EntityTestNoLoadStorage.php

EntityTestUpdateStorage.php in core/modules/system/tests/modules/entity_test_update/src/EntityTestUpdateStorage.php

FeedStorage.php in core/modules/aggregator/src/FeedStorage.php

FieldDataCountTest.php in core/modules/field/tests/src/Kernel/FieldDataCountTest.php

FileStorage.php in core/modules/file/src/FileStorage.php

ItemStorage.php in core/modules/aggregator/src/ItemStorage.php

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

MenuLinkContentStorage.php in core/modules/menu_link_content/src/MenuLinkContentStorage.php

NodeStorage.php in core/modules/node/src/NodeStorage.php

PathAliasStorage.php in core/modules/path_alias/src/PathAliasStorage.php

SqlContentEntityStorageTest.php in core/tests/Drupal/Tests/Core/Entity/Sql/SqlContentEntityStorageTest.php

Contains \Drupal\Tests\Core\Entity\Sql\SqlContentEntityStorageTest.

system.install in core/modules/system/system.install

Install, update and uninstall functions for the system module.

taxonomy.module in core/modules/taxonomy/taxonomy.module

Enables the organization of content into categories.

TermStorage.php in core/modules/taxonomy/src/TermStorage.php

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

views.views.inc in core/modules/views/views.views.inc

Provide views data that isn't tied to any other module.

ViewsEntitySchemaSubscriber.php in core/modules/views/src/EventSubscriber/ViewsEntitySchemaSubscriber.php

WorkspaceAssociation.php in core/modules/workspaces/src/WorkspaceAssociation.php