The retention feature is designed to prevent files from being lost from the system forever (being purged from Trash) but does not prevent users from deleting files. Deleted files will remain within trash while they are subject to retention and cannot be purged; manually or automatically.
When the retention period expires, deleted files can be purged manually or will be automatically purged according to your configured trash purge interval. For retained files that have not yet been deleted, nothing happens to them at the point of retention expiry.