To have actual off the record, you need disappearing messages (and trust for your colleagues) like in Signal. Not simply paywalled messages.

Oh sure, the message history part 90 day is safe only from casual inquiry, not a determined adversary. it's like the first kind of cryptography not the second (1)

I'll take better suggestions?

1) https://libquotes.com/bruce-schneier/quote/lbx8x8g

It isn't "90 days of history". It's 90 days of history you can see. Slack retains it indefinitely.