You’re right, the UI is misleading here.
The Generics field is available for ordinary classes and interfaces, and the Deprecated checkbox is also available. The field labelled Message only becomes enabled after Deprecated is selected because it contains optional deprecation guidance, such as “Use NewThing instead.” It isn’t a general class description.
I’ve renamed it Deprecation message throughout Studio and updated the documentation. The change will be included in the next release: Deprecation message fields are labelled clearly.
For documenting evolving design thoughts, a Note is currently the right choice. Member descriptions are also available for properties, constants, methods, and events, but Studio doesn’t currently provide an equivalent description field for the class itself.