There's a notify-type-availability property on individual notification types, but it doesn't work most of the time. Type availability is set on the category level, except for hidden categories (not visible in preferences), where it's set on the notification type level. This leads to confusion like in this patch, where we tried to set notify-type-availability on a notification type because it looked like it would work (other types were doing it), but then to everyone's surprise it didn't work.
Let's make this consistent by setting this property only on categories, never on individual types. This may require splitting existing hidden categories.