Hacker News Reader

There is an open source alternative to Slack that can be self hosted called Rocket Chat: https://rocket.chat/

You can self host mattermost, which has been working well for us so far.

To my knowledge no. That is going to be an issue for current users of Hipchat Server/Data Center.

I've seen a private Slack instance at SAP, so it seems to be possible for big corps..

Maybe it'll be a good opportunity to push for a self-hosted Matrix homeserver...