The module is not working OOTB since it throws an exception upon the first save on the maintenance form.
Error message:
Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for readonlymode.settings with the following errors: readonlymode.settings:forms.additional.edit variable type is string but applied schema class is Drupal\Core\Config\Schema\Sequence, readonlymode.settings:forms.additional.view variable type is string but applied schema class is Drupal\Core\Config\Schema\Sequence in Drupal\Core\Config\Development\ConfigSchemaChecker->onConfigSave() (line 95 of /mnt/files/local_mount/build/web/core/lib/Drupal/Core/Config/Development/ConfigSchemaChecker.php).
Install 8.x-1.1, enable the module, navigate to the site maintenance page, enable read-only mode. Notice the exception/error.
Fix the config schema.
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Fixed
1.1
Code
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Issue #3224538 by attila.fekete: Config schema errors
Automatically closed - issue fixed for 2 weeks with no activity.