Per a team member, maven.repo.local "is supported by the MavenCli class which uses the value to set the local repository. But beyond multiple references in StackOverflow and the Maven JIRA, I don't see it referenced in any official documentation for Maven 3.x, just Maven 1.x."
It may be that we need a similar page for Maven 3.x. There could well be other properties that should be documented too. I don't see it here, for example: https://cwiki.apache.org/confluence/display/MAVEN/Maven+Properties+Guide