6 string references to maintenance_mode in Drupal 9

DbUpdateController::batchFinished in core/modules/system/src/Controller/DbUpdateController.php

Finishes the update process and stores the results for eventual display.

DbUpdateController::triggerBatch in core/modules/system/src/Controller/DbUpdateController.php

Starts the database update batch process.

SiteMaintenanceModeForm::submitForm in core/modules/system/src/Form/SiteMaintenanceModeForm.php

Form submission handler.

UpdateReady::submitForm in core/modules/update/src/Form/UpdateReady.php

Form submission handler.

update_authorize_install_batch_finished in core/modules/update/update.authorize.inc

Implements callback_batch_finished().

update_authorize_update_batch_finished in core/modules/update/update.authorize.inc

Batch callback: Performs actions when the authorized update batch is done.