9 files declare their use of MessengerTrait in Drupal 9

BlockPluginTrait.php in core/lib/Drupal/Core/Block/BlockPluginTrait.php

ControllerBase.php in core/lib/Drupal/Core/Controller/ControllerBase.php

DefaultParser.php in core/modules/aggregator/src/Plugin/aggregator/parser/DefaultParser.php

EntityListBuilder.php in core/lib/Drupal/Core/Entity/EntityListBuilder.php

FormBase.php in core/lib/Drupal/Core/Form/FormBase.php

FormErrorHandler.php in core/lib/Drupal/Core/Form/FormErrorHandler.php

MailManager.php in core/lib/Drupal/Core/Mail/MailManager.php

MigrationState.php in core/modules/migrate_drupal/src/MigrationState.php

PluginBase.php in core/lib/Drupal/Core/Plugin/PluginBase.php