We originally used Ant for tar/untar functionality, but we migrated to Commons Compress in https://github.com/jenkinsci/jenkins/pull/1670, which also added the Commons Compress library to the WAR. Fast forward to today, and recent releases of Commons Compress depend on Commons Lang 3. We don't want to keep increasing the API surface area of the WAR by pulling in these random libraries. And adding Commons Lang 3 to the WAR would override the library plugin we ship today.

A solution was found in 2.460 by migrating from Commons Compress back to Ant. Our original reason for migrating away from it was that it did not support large files > 8 GiB in size. Today, it does, as verified with an automated test. We are unlikely to ever remove Ant from Jenkins core, as it is used in a lot of places in core, so why not use it for tar/untar functionality as well. And unlike Commons Compress, it has no dependency on Commons Lang 3.

In the meantime, we are still blocked from upgrading to the latest release of Commons Compress, because the latest release depends on Commons Lang 3, which we do not want to add to the WAR. But the release of 2.460 shows a path forward. With core's dependency on Commons Compress removed, plugins can be adapted, and we can eventually remove Commons Compress from core.

Plugins can be adapted by either (in the vast majority of simple cases) migrating to core's copy of Ant or (in the case of e.g. pipeline-utility-steps and jobcacher-plugin, both of which use Commons Compress features that are not currently available in Ant) a new Commons Compress library plugin could be created and plugins migrated to that instead. That Commons Compress library plugin could depend on Commons Lang 3 as a plugin-to-plugin dependency.