Next-Gen kanban boards currently auto-clear items from the "Done" column after 14 days. There are a handful of different threads (notably https://community.atlassian.com/t5/Next-gen-questions/How-to-clear-old-items-out-of-the-quot-Done-quot-column/qaq-p/852627) that touch on the topic of how to alter/customize this behavior, because it doesn't suit many users' needs.
The common issue is that the "Done" column quickly grows to an unmanageable size (ours consistently sits at around 50+ items), and makes the column unrepresentative of the current iteration. The "Backlog" view also uses the same constraint, thus making managing the Board/Backlog more difficult.
The Jira roadmap shows no indication of this even being on Jira's radar, despite numerous complaints about it. The issue is close to becoming a dealbreaker for our continued use of Jira, so it would be great if someone (from Jira) could offer feedback on whether there is anything planned for how to address this.