The CREATE TABLE documentation groups NOT NULL under table_constraint.

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/18/ddl.html
Description:

Unlike UNIQUE or CHECK, NOT NULL cannot be used standalone at the table
level in a standard way
Example: CREATE TABLE t (a int, NOT NULL a); fails.
It only works as an "out-of-line" constraint in very specific inheritance
scenarios.