The @PLUGIN@ plugin is configured in the $site_path/etc/@PLUGIN@.config
file:
[readonly] message = Gerrit is down for maintenance
If the configuration is modified, the plugin must be reloaded for the changes to be effective.
readonly.message
: Message to be shown to clients when attempting to perform an opeation that is blocked due to the server being in read-only mode. When not specified, the default is “Gerrit is under maintenance - all data is READ ONLY”.