question

A primary key column might unexpectedly contain null values if the primary key is disabled.

This oracle documentation says that a primary key can contain null values. But if you try to create a table with a primary key defined this column will always be not nullable.

-- Create Table
CREATE TABLE a (a number primary key, b number);
-- Check if constraints are there
SELECT table_name, constraint_name, constraint_type FROM user_constraints 
WHERE table_name = 'A';
-- Describe Table
DESC a
-- Disable Constraint
ALTER TABLE a disable constraint xxxx; -- Fill in the right name
-- Describe Table again
DESC a

If you disable the constraint the column will be nullable.
Disable a constraint has no impact if the constraint is based on an index. The index will remain and support your queries.
If this index is a unique index it becomes curious. You can have more than one row having null values into your "primary key" columns. An oracle unique index checks for uniqueness of not null values.

-- Create Table
CREATE TABLE a (a number primary key, b number unique);
-- Check if constraints are there
SELECT table_name, constraint_name, constraint_type FROM user_constraints 
WHERE table_name = 'A';
-- Insert data
INSERT INTO a VALUES (1, 2);
INSERT INTO a VALUES (2, null);
INSERT INTO a VALUES (3, null);
-- Check data
SELECT * FROM a;

Unexpected.