I actually think slack is both an example of poor design and good design.

While the multi-tenant with zero shared ui elements is a monstrosity, I think they do a really good job at discoverability. One of the things application level menus aren’t great at is connecting what element must be focused to take the action. In something like word, it’s all a document so most things are applicable most of the time, but that breaks down in chat like slack where there is a notion of chats, files, links, users, etc and the localized contextual controls while not always great aren’t terrible either.