I had to manage a Slack migration to another org we were merging with. As the owner, normally I couldn’t even see the names of private channels, but when it came down to the migration, upgrading the account to Business+ tier, the full export included everything (private channels and DMs), which we imported into the new org.

Slack sends notifications to all Admins that the export was happening, and i’ve only seen that notification once after using Slack for 10 years.