27 files declare their use of SelectInterface in Drupal 9

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

block_content.module in core/modules/block_content/block_content.module

Allows the creation of custom blocks through the user interface.

Cache.php in core/lib/Drupal/Core/Cache/Cache.php

CommentSelection.php in core/modules/comment/src/Plugin/EntityReferenceSelection/CommentSelection.php

Condition.php in core/lib/Drupal/Core/Entity/Query/Sql/Condition.php

Condition.php in core/lib/Drupal/Core/Entity/Query/Sql/pgsql/Condition.php

ConditionAggregate.php in core/lib/Drupal/Core/Entity/Query/Sql/ConditionAggregate.php

database.api.php in core/lib/Drupal/Core/Database/database.api.php

Hooks related to the Database system and the Schema API.

FieldOrLanguageJoin.php in core/modules/views/src/Plugin/views/join/FieldOrLanguageJoin.php

JoinPluginBase.php in core/modules/views/src/Plugin/views/join/JoinPluginBase.php

MenuTreeStorage.php in core/lib/Drupal/Core/Menu/MenuTreeStorage.php

node.module in core/modules/node/node.module

The core module that allows content to be submitted to the site.

Node.php in core/modules/node/src/Plugin/migrate/source/d6/Node.php

Node.php in core/modules/node/src/Plugin/migrate/source/d7/Node.php

NodeComplete.php in core/modules/node/src/Plugin/migrate/source/d7/NodeComplete.php

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

NodeRevision.php in core/modules/node/src/Plugin/migrate/source/d6/NodeRevision.php

Query.php in core/lib/Drupal/Core/Entity/Query/Sql/Query.php

SearchQuery.php in core/modules/search/src/SearchQuery.php

SelectionInterface.php in core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionInterface.php

SelectionPluginBase.php in core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php

SqlBaseTest.php in core/modules/migrate/tests/src/Kernel/SqlBaseTest.php

Contains \Drupal\Tests\migrate\Kernel\SqlBaseTest.

Tables.php in core/modules/workspaces/src/EntityQuery/Tables.php

Tables.php in core/lib/Drupal/Core/Entity/Query/Sql/Tables.php

Update.php in core/lib/Drupal/Core/Database/Driver/pgsql/Update.php

UserSelection.php in core/modules/user/src/Plugin/EntityReferenceSelection/UserSelection.php

views_test_query_access.module in core/modules/views/tests/modules/views_test_query_access/views_test_query_access.module

Module to test entity query access in Views.