As of December 21, the https://oss.sonatype.org/content/repositories/plexus-snapshots URL produces an HTTP error 503, which cause the src/it/MCOMPILER-522-unresolvable-dependency/ integration test. That test contains the following dependency:

<annotationProcessorPaths>
  <path>
    <groupId>org.apache.maven.plugins.compiler.it</groupId>
    <artifactId>annotation-processor-non-existing</artifactId>
    <version>1.0-SNAPSHOT</version>
  </path>
</annotationProcessorPaths>

Because that dependency is not found on Maven central, it was searched on Plexus snapshots.