Slack works because of push notifications. A random IRC app can't handle push notifications for you because they aren't the IRC server and aren't running a bouncer for you. On the other hand, the IRC server can't handle push notifications for you because they don't have an app signed with Apple's key sending the notifications.
Basically protocols as a general thing can't work on iOS, only individual apps/services which get to read all your messages and access your login credentials work.
> Basically protocols as a general thing can't work on iOS, only individual apps/services which get to read all your messages and access your login credentials work.
Which I think is a reasonable requirement for a chat service, especially on mobile.
Basically protocols as a general thing can't work on iOS, only individual apps/services which get to read all your messages and access your login credentials work.