Registered shares are essentially shares offered for sale by the company and certain insiders. Unregistered shares are everything else. Slack was probably selling shares on behalf of non-executive shareholders for unregistered part.
So it sounds like IPOs are basically preventing employees from dumping and it also sounds like other non-employees or registered shares can sell. Directly listing sounds a lot more fair than IPOs.
Fair us what a contract says is fair. (not serious here, just making a point)