16 files declare their use of AnonymousUserSession in Drupal 9

ActiveLinkResponseFilterTest.php in core/tests/Drupal/Tests/Core/EventSubscriber/ActiveLinkResponseFilterTest.php

AnonymousUserSessionTest.php in core/tests/Drupal/Tests/Core/Session/AnonymousUserSessionTest.php

CommentFieldAccessTest.php in core/modules/comment/tests/src/Kernel/CommentFieldAccessTest.php

CommentItem.php in core/modules/comment/src/Plugin/Field/FieldType/CommentItem.php

CommentLinksTest.php in core/modules/comment/tests/src/Kernel/Views/CommentLinksTest.php

CommentUserNameTest.php in core/modules/comment/tests/src/Kernel/Views/CommentUserNameTest.php

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

DisplayPageTest.php in core/modules/views/tests/src/Kernel/Plugin/DisplayPageTest.php

EntityAccessControlHandlerTest.php in core/tests/Drupal/KernelTests/Core/Entity/EntityAccessControlHandlerTest.php

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

FilterAPITest.php in core/modules/filter/tests/src/Kernel/FilterAPITest.php

FormCacheTest.php in core/tests/Drupal/KernelTests/Core/Form/FormCacheTest.php

HandlerFilterCurrentUserTest.php in core/modules/user/tests/src/Kernel/Views/HandlerFilterCurrentUserTest.php

UiHelperTrait.php in core/tests/Drupal/Tests/UiHelperTrait.php

UpdateKernel.php in core/lib/Drupal/Core/Update/UpdateKernel.php

user.module in core/modules/user/user.module

Enables the user registration and login system.