Several IRC servers do have support for authentication and access control (and audit trails as well I suppose).
Only centralized history/logging and search would need to be bolted on if needed.
In the non-centralized case your IRC client takes care of all of that.
For business users, there are regulatory requirements. You need to keep information around for some period of time, but not forever. History and searching is useful for spreading tribal knowledge throughout an organization.
Does that actually extend to Slack/slack-like things though?
Since I would see Slack more of a replacement for phone calls or hallway discussions.
Neither of which typically has any logs or recordings (and I wouldn't want to work somewhere that did keep such logs).
In what areas would you find such requirements? And shouldn't the default position be that it is illegal to keep those logs? Especially those involving direct messages between employees.
Only centralized history/logging and search would need to be bolted on if needed. In the non-centralized case your IRC client takes care of all of that.