Currently deprecation_logger.deprecated() writes deprecation announcement to logstash-deprecation.log. However, such important information never shows in regular log logstash-plain.log, and gets overlooked by users. Logstash should write the deprecation message to both standard logger and deprecation logger.

We should review the usage of deprecation_logger.deprecated() in core and plugins. Some are logged in both logger.warn() and deprecation_logger. [1]. Once the logger has wired back to standard logger, we should not duplicate the log with warn()