9 files declare their use of ConfigEntityUpdater in Drupal 9

action.post_update.php in core/modules/action/action.post_update.php

Post update functions for Action module.

ConfigEntityUpdaterTest.php in core/tests/Drupal/KernelTests/Core/Config/Entity/ConfigEntityUpdaterTest.php

file.post_update.php in core/modules/file/file.post_update.php

Post update functions for File.

layout_builder.post_update.php in core/modules/layout_builder/layout_builder.post_update.php

Post update functions for Layout Builder.

system.post_update.php in core/modules/system/system.post_update.php

Post update functions for System.

text.post_update.php in core/modules/text/text.post_update.php

Contains post update hooks for the text module.

tour.post_update.php in core/modules/tour/tour.post_update.php

Post update functions for Tour.

user.post_update.php in core/modules/user/user.post_update.php

Post update functions for User module.

views.post_update.php in core/modules/views/views.post_update.php

Post update functions for Views.