How to Integrate Slack & WhatsApp on Arch Linux Guide
How to Install WhatsApp and Slack desktop Chat Messenger Client in One App on Arch GNU/Linux desktop – Step by step Tutorial.
And to Install WhatsApp & Slack Client on Arch Linux you dispose of some cool Alternatives:
Especially relevant: currently Ferdi is the only completely Free alternative, and so I listed them in order of my Personal Testing Preference.

WhatsApp & Slack Client Arch Linux Installation Guide