On July 18, 2022, Slack announced that starting September 1, search history for organizations on Slack’s Free plan will be limited to just the past 90 days of message history. Instead of a 10,000-message limit and 5 GB of storage, we are giving full access to the past
see my reply above. zulip is less of a slack clone than mattermost or rocketchat are; it has “topics” within “streams” which are a kind of threading.
besides database backups it can also export everything to static html for archival.