Not to mention that while Slack still has a web interface, they can't really expect to control what extensions are installed in the user's browser and how they render/modify the data the website is sending them. That'd be like Reddit or Google sending uBlock Origin a C&D because it can block ads on their sites. Or SalesForce C&D'ing whoever did an extension that implemented XKCD's force/horse replacement (https://xkcd.com/1418/).

Slack can shut off their web interface and rely on their own apps if they want to control that stuff.