우선 전제조건으로 정렬순서가 정해져야만 가능합니다.
임의로 seq 컬럼을 추가하여 처리했습니다.

WITH t AS
(
SELECT 1 seq, '6.' id, 1 depth FROM dual
UNION ALL SELECT 2, '6.1'  , 2 FROM dual
UNION ALL SELECT 3, '7.'   , 1 FROM dual
UNION ALL SELECT 4, '7.1'  , 2 FROM dual
UNION ALL SELECT 5, '9.'   , 1 FROM dual
UNION ALL SELECT 6, '9.1'  , 2 FROM dual
UNION ALL SELECT 7, '9.1.1', 3 FROM dual
UNION ALL SELECT 8, '10.1' , 3 FROM dual
UNION ALL SELECT 9, '10.2' , 2 FROM dual
)
SELECT seq
     , id
     , depth
     , (SELECT MAX(id) KEEP(DENSE_RANK LAST ORDER BY seq)
          FROM t
         WHERE seq   < a.seq
           AND depth = a.depth - 1
        ) parent_id
  FROM t a
 ORDER BY seq
;

급한관계로 지저분하게 처리는 했는데

역시 깔끔한 쿼리 감사합니다.