If your security depends on people not using browser features, maybe don’t use a browser then. Ultimately, if your security depends on the client being unmodified, you basically lost. You can make it harder for an attacker, but that’s the problem DRM tries to solve (and consistently fails to)
But that's utterly ridiculous. All the data Slack sends to my browser is already mine to view, so if there's an issue of "privacy and security", it's on their end.