Please note that, technically, you already can delecte users and their tickets via console, see:
https://docs.zammad.org/en/latest/console/dangerzone-for-experts.html#delete-one-or-more-users-with-all-their-related-information
As for the UI part of this functionality, you can find the issue to this here:
It’s currently within QA and should appear with Zammad 3.2 which has no release date yet.