Expensive in which way? When an association is accessed and the referenced records cannot be found, we'd apply the same behavior as if @NotFound(action = NotFoundAction.IGNORE) was explicitly given. Where would be the extra cost? I'd do it in the case were the associating entity is loaded/accessed anyways.

Yes, probably we'd need changes in ORM to get it implemented. An option to enable it may be a way, though to me it's more dependent on the dialect, e.g. in MongoDB you always need to take into account stale references, so IMO it should be enabled on that datastore by default.