(defclass always-bound-class (standard-class) ())

(defmethod sb-mop:validate-superclass ((class always-bound-class)
                                       (superclass standard-class))
  t)

(defmethod sb-mop:slot-makunbound-using-class
    ((class always-bound-class) object slotd)
  (error "The slot ~S must be always bound in object ~S."
         (sb-mop:slot-definition-name slotd) object))

(defclass test-class ()
  ((slot :initarg :slot))
  (:metaclass always-bound-class))

(slot-makunbound (make-instance 'test-class :slot 42) 'slot)