Slack is not available as a self-hosted solution but there are plenty of alternatives for power users and business that want to host a solution on-premise. The best Self-Hosted alternative is Mattermost, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Slack and many of them is available as a self-hosted solution so hopefully you can find a suitable replacement. Other interesting Self-Hosted alternatives to Slack are Element (Free Personal, Open Source), Zulip (Free Personal, Open Source), Rocket.Chat (Freemium, Open Source) and Matrix.org (Free, Open Source).
The future of Team collaboration. Secure. Cross platform. Manage Teams, Groups, Private & Public Channels. Invite. Meet. Work. Automate with over 700'000 Apps. Built-in Webinar & Webmeetings. Available as PrivateCloud or On Premise. Decentralized.
PrivMX is safe and easy collaboration software for all kinds of teams, with virtual workspace protected by end-to-end encryption. Each team uses desktop/laptop client software and its own (zero-knowledge) PrivMX Team Server.
Conduit is a simple, fast and reliable chat server powered by Matrix. Conduit is an alternative to Synapse and tries to be lightweight and easy to install, but it is still in development. What is Matrix?