Starting August 26, 2024, Slack will delete content older than one year from free workspaces, they will still access the last 90 days of history. Users must upgrade to a paid plan to keep older data before the policy change.
https://alternativeto.net/news/2024/8/slack-will-delete-content-older-than-one-year-for-free-plan-users-starting-august-26th/