Slack is not end-to-end encrypted, creating a pile of sensitive data on Slack’s servers. Data is vulnerable to nation-state hackers, lawfare attacks by the U.S. government (think PRISM) and malicious insiders. Wickr is not open source, although it has published its cryptography code for public inspection. Keybase Teams uses NaCl under the hood and not PGP, but by default, it offers a self-destruct message feature that uses ephemeral keys with forward secrecy.”]
Source: https://www.csoonline.com/article/3332686/three-encrypted-slack-alternatives-worth-a-look.html