Very confusingly, the keys to this are both categories and types, without any way of distinguishing.
In practice, we have do have categories and types with the same name. Not only that, but sometimes they are not one-type categories. E.g. the 'edit-thank' category has the 'edit-thank' and 'flow-thank' types. There is no way of specifying behavior for the type without accidentally affecting the category.
https://phabricator.wikimedia.org/diffusion/ECHO/browse/master/includes/controller/NotificationController.php;db8728aad053f1b9d120588fd072048d517e68fc$143 (note the parameter to getEventNotifyTypes comes from getType, not getCategory.
'all' is not that big a problem, but if we're breaking compatibility anyway, we might as well separate this out.