Starting with version 7.10, Jira Software Data Center allows administrators to archive projects.
The archived issues become read-only and can only be accessed through a direct link. They are removed from the Lucene index and will not be a part of a search result even if they match the search criteria.
Archived issues will not be visible in Structure – even if they were added manually, and even in the archived structures.
When you archive a project, check for structures that have issues from that project. Contact the users to see if they are ok with the archived issues disappearing from the structures.
When you restore a project from archive, you run a project reindex. After the reindex has finished, the issues should reappear in Structure.
Since Structure has internal caching of which issues are visible to which users, it might take a few minutes after the reindex' completion before issues are shown in Structure. To force clearing of caches and to see the restored issues in structures immediately, use "force reload" action from your browser (Shift+Reload).