How to reclaim the space of dropped columns of a table?

"To force immediate reclamation of space occupied by a dropped column, you can execute one of the forms of ALTER TABLE that performs a rewrite of the whole table. This results in reconstructing each row with the dropped column replaced by a null value."

This seems to a bit vague for users (how to rewrite but keep the table definition) and it seems to still keep the dropped columns (though with null). Isn't it better to leave the functionality to command like 'vacuum full' to completely remove the dropped columns (i.e. no dropped columns in pg_attributes and no null values for dropped columns for a table)?