There is no API for accessing optional/required properties. The only thing it offers is CXCursor_ObjCPropertyDecl. But in Protocol we can have both optional/required properties. So there is still lack of API.
@property (readonly) NSString *string1;
@property (readonly) NSString *string2;
We do not have separate ASTs for such things as @optional/@required. Such info is kept in AST for the properties and methods.
Lack of CXCursor for them could be fall out from that.
A function could be added to query the optional/required state of the property.
Please either file an enhancement request in bugzilla, or even better, see if you can provide a patch for reviewing.