It says they're "irretrievable", which isn't necessarily the same thing. That you can't get that data back doesn't mean it's no longer stored by Slack in a way that a sufficiently severe compromise of their infrastructure might reveal.

I doubt they keep customer data after you're not a customer anymore. Sure, they could be lying but we don't have information pointing to that, do we?

They are subject (or have subjected themselves) to various security standards: https://slack.com/intl/en-ie/security

And their security whitepaper [0] mentions:

Customer data is removed immediately upon deletion by the end user or upon expiration of message retention as configured by the customer administrator

It would be a stretch to say a message is deleted permanently but a workspace is kept forever, for unknown reasons. Occam's razor and stuff.

0 - https://a.slack-edge.com/78b2/marketing/downloads/security/S...

It probably means the data is encrypted at rest and they discard the encryption key. Of course I doubt we'll ever know because they won't give us that level of detail unless they are compelled to.

At the time, there were reasons we couldn't completely delete the workspace. I believe there was certain data we wanted retained, and other data we wanted removed.