MNG-4162, a bunch of reporting logic was removed from Maven core.
This was globally successful, but not on one topic: it was expected to remove pom's reporting section and replace it with a normal reportPlugins parameter in maven-site-plugin configuration.
We later discovered that with this normal parameter, we missed one crucial feature: report plugins inheritance, which would require a new mechanism MSITE-484
Then we reverted our instructions in maven-site-plugin documentation
MSITE-647: the new reportPlugins parameter is now just an implementation detail, but should not be used by end-users, reporting section remains the way to configure report plugins MSITE-684
Now adding a warning in Maven core when it detects a reportPlugins parameter in a maven-site-plugin configuration is easy to do, will help users come back to normal reporting configuration, and will permit future enhancements (we need to finally completely clean-up the situation we created with this story...)