I had raised a PR https://github.com/jenkins-infra/update-center2/pull/738 to remove old versions of the plugin, the PR was closed with a comment from danielbeck  .

We would like to remove the old versions of the plugin as from a Delphix customer and security standpoint, the recommendation is to remove these plugin versions. We see a risk of exposing internal credentials, which we do not want to take. Furthermore, the data shows that neither of these two plugin versions has been downloaded, giving us confidence to remove them at this early stage.