set termout off prevents text (results of queries, prompt etc.) from being written (echo'ed) to the terminal if the text originates from a statement in an SQL script started with start, @ or @@.
Output whose origin is a statement that is typed interactively or redirected (STDOUT | STDIN) to SQL*Plus is not affected by the value of termout.
In order to display text again, set termout on must be used.