From ArchWiki

Slack is a proprietary web-based chat program aimed at corporate users with a freemium subscription model.

Installation

It is available via snap and AUR: slack-desktopAUR and slack-electronAUR using the electron system package instead of the bundled one.

Troubleshooting

Login fails silently

You need to install Firefox either firefox if you installed via AUR or as a snap if Slack is installed as snap and login using this specific browser.