Old Javadoc files are not removed, so they can still be found by direct URL access (e.g. from Search engines)
To reproduce:

• Have a simple Maven project with one class "A.java".
• Setup a M2/M3 Job in jenkins. Use "clean compile javadoc:javadoc" as goals
• Run the job. As expected, Javadoc is created and a Javadoc link appears.
• Rename the file/class to "B.java"
• Run the job again.
  As expected, the Javadoc is updated showing the new B-class.
  BUT: in the Javadoc directory of the build-job (jobs/<name>/modules/<module$name>/javadoc/) there is still A.html around. So while it is no longer linked from the index pages, the file itself is still there. Opening http://jenkins-url/job/<name>/javadoc/A.html in a browser still shows some (now outdated) content.

In my opinion, old/outdated Javadoc files should be removed. I assume that currently all files are copied over from the workspace's target/site/apidocs directory, overwriting existing files. But old ones are not removed.