Slots are not directly assigned to content types. Rather they are associated with templates which then are associated with content types. But it would be useful if you could view a list of all the slots that a user might be asked to fill for an item of a given content type. At the moment when, for example, deciding which slots to add to a new template for a content type, you have to open every existing template for that content type to see what is already in use.
My suggestion is a new subfolder inside each content type in the Content Design tab, called something like “Associated slots”. You wouldn’t be able to drag-and-drop slots into this folder, but then there are already lots of things in the Workbench you cannot drag-and-drop to.