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I think they retain it but you can't access it. One Slack I'm in activated the trial and everything was there and searchable.

There used to be a workaround where you can still access the full archive as an exported JSON document

Seems to be still available for "Workspace Owners/Admins and Org Owners/Admins" https://slack.com/help/articles/201658943-Export-your-worksp...