Having loosely followed this discussion, the way I'm thinking of ‘closed’ is as a modifier which would let you switch over something from outside the module in which it is declared.
From outside the declaring module:
• A closed enum's cases can be exhaustively switched.
• A closed protocol's conforming types can be exhaustively switched.
• A closed class's subclasses can be exhaustively switched.
If this is correct, I can't help but think ‘closed’ is describing something subtly different in each case - picking and choosing the ‘important’ relationship for each type, while protocols already have a subtyping relationship, and it sounds like there's possibility for enum subtyping in the future.
I'd rather keep ‘open’ (and a potential future ‘closed’) purely describing the subtyping relationship, and have some other means of labelling conformance and cases as switchable.