The book states the following:

The main difference for domains compared with enums is that the NOT NULL constraint portion is part of the domain.

This is incorrect. The domain has a CHECK constraint for valid non-NULL values, but the NOT NULL constraint is part of the column declaration and can be removed independently of the domain, as follows:

owner@localhost:5432 rideshare_development# ALTER TABLE vehicles ALTER COLUMN status DROP NOT NULL;
ALTER TABLE
owner@localhost:5432 rideshare_development# UPDATE vehicles SET status = NULL;
UPDATE 4
owner@localhost:5432 rideshare_development# SELECT DISTINCT(status) from vehicles;
 status
--------

(1 row)

owner@localhost:5432 rideshare_development# \d vehicles
                                        Table "rideshare.vehicles"
   Column   |              Type              | Collation | Nullable |               Default
------------+--------------------------------+-----------+----------+--------------------------------------
 id         | bigint                         |           | not null | nextval('vehicles_id_seq'::regclass)
 name       | character varying              |           | not null |
 created_at | timestamp(6) without time zone |           | not null |
 updated_at | timestamp(6) without time zone |           | not null |
 status     | vehicle_statuses               |           |          | 'draft'::text
Indexes:
    "vehicles_pkey" PRIMARY KEY, btree (id)
    "index_vehicles_on_name" UNIQUE, btree (name)
Referenced by:
    TABLE "vehicle_reservations" CONSTRAINT "fk_rails_7edc8e666a" FOREIGN KEY (vehicle_id) REFERENCES vehicles(id)

owner@localhost:5432 rideshare_development# \dD vehicle_statuses
                                                           List of domains
  Schema   |       Name       | Type | Collation | Nullable | Default |                             Check
-----------+------------------+------+-----------+----------+---------+---------------------------------------------------------------
 rideshare | vehicle_statuses | text |           |          |         | CHECK (VALUE = ANY (ARRAY['draft'::text, 'published'::text]))
(1 row)