The settings.xml file points to http://repo1.maven.org/maven2, which has seemed to go away. Thus, these warnings get produced when compiling:

[WARNING] Could not transfer metadata [org.onap.policy.models:policy-models-decisions:2].2.1-SNAPSHOT/maven-metadata.xml from/to central (http://repo1.maven.org/maven2/): Failed to transfer file http://repo1.maven.org/maven2/org/onap/policy/models/policy-models-decisions/2.2.1-SNAPSHOT/maven-metadata.xml with status code 501
[WARNING] Failure to transfer [org.onap.policy.models:policy-models-decisions:2].2.1-SNAPSHOT/maven-metadata.xml from http://repo1.maven.org/maven2/ was cached in the lo

Via the browser, looks like https is now required:

501 HTTPS Required. Use https://repo1.maven.org/maven2/ More information at https://links.sonatype.com/central/501-https-required

Tested locally, the warning seems to disappear.