The system property 'hudson.model.ParametersAction.keepUndefinedParameters' was updated to take a 'false' value, with the intent of supressing warnings about undefined parameters that are being ignored. This behavior is documented in https://wiki.jenkins.io/display/JENKINS/Features+controlled+by+system+properties

However, the check implement actually does the reverse, and only shows the warning messages if the system property is set to false (https://github.com/arothian/jenkins/blob/master/core/src/main/java/hudson/model/ParametersAction.java#L329)