I get it from the perspective of slack is a disorganized mess in terms of long term history, but moving sensitive data from one system to another with similar security doesnt seem like a win (or loss) for security to me.
Note I didn’t call out sensitive information specifically. Just that you wished you deleted messages. You shouldn’t store secrets in a ticket system either.
I’m saying it’s ok for quick cursory discussions but those who retain Slack messages for tribal knowledge tend to have larger organizational issues. This knowledge should be in tickets where it’s expected to write well written responses with the intention of documenting knowledge.