Fixed, it works as expected for Backupery for Slack Workspace v. 1482 and above.
Please note, the issue is related to Backupery for Slack Workspace only, Backupery for Slack Personal works as expected. After adding Backupery for Slack Workspace to a private channel that was public once and is private now you may notice that the channel isn’t exported as expected. It’s a known issue that is related to Slack API Framework. Slack doesn’t have plans to make changes to the current app framework since they decided to deprecate it in favor of the new app framework which is currently in development. There are alternative workarounds to export these channels: Install Backupery for Slack Personal on behalf of the user who has access to these private channels. As Backupery for Slack Personal exports the data which is related to this user, it will also export these private channels. Create a new Slack member and add it to the required private channels. Then install and run Backupery for Slack Personal on behalf of this member.