15 files declare their use of SqlContentEntityStorageSchema in Drupal 8

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

ContentModerationStateStorageSchema.php in core/modules/content_moderation/src/ContentModerationStateStorageSchema.php

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

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

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

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

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

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

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

schema.inc in core/includes/schema.inc

Schema API handling functions.

SchemaLegacyTest.php in core/tests/Drupal/KernelTests/Core/Database/SchemaLegacyTest.php

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

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

Install, update and uninstall functions for the system module.

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

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