SOLR-10966 we added a "bad" workaround to prevent Hadoop's Shell class from breaking with Java 9 (StringIndexOutOfBoundsException). This was resolved in Hadoop 2.7.4, released a few weeks ago. We should revert the bad hack and update Hadoop.
After running tests, I see no issue with the bugfix relaese.
I will commit to master and 7.x and once it settled, I will backport. If we can't get this into 6.6.1, it's not so bad, but then we have to live with the "bad" hack there.