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...