|rm -rf ~/Library/Application\ Support/Slack/|
|rm -rf ~/Library/Containers/com.tinyspeck.slackmacgap/|
|rm -rf ~/Library/Preferences/com.tinyspeck.slackmacgap.plist|
|rm -rf ~/Library/Saved\ Application\ State/com.tinyspeck.slackmacgap.savedState|
From 10.11 onwards, macOS protects certain folders by using SIP (System Integrity Protection). It may not be able to 'rm' the kext from terminal.
I still use the manual way to remove the Slack desktop app from Mac, following these steps: