That's my question.Do you have to be able to play it by memory.Must it be played flawless, with all repeats, dynamics...Must the pulse be metronome correct ?Is not the least mistake forgiven ?Is this different from person to person ? Is this different for a stage piece or one you play at home ?Who is the final judge when there is no teacher.My previous teacher moved on to another piece while he was sure I think I did not played the 1st mov of the Ravel sonatine as he should. Can you give yourself quotes on a performance, 10/20 , 12, ...I wonder....
Further, Arthur (Artur!) Rubinstein never had a piano lesson after the age of sixteen. So, that means that the over 200 recording sessions he had later were also sight read.
How on earth does "no lessons" = "must be sightread"?