The Cleanup pause in G1 historically used to do a lot of remembered set cleanup, weeding out obsolete entries and doing not much else than preparing for the mixed gcs.

In JDK11 the remembered set cleanup had become obsolete (concurrently rebuilding remembered sets), the pause now does not do anything worth "Cleanup".

At the same time there is concern for confusion with the "Concurrent Cleanup" phase to prepare the bitmap after, so it would be good to rename the pause to something more specific.