The best open source alternative to Slack 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 open source so hopefully you can find a suitable replacement. Other interesting open source 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).
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.
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.