Oracle 18c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE mytable(
   CODE1 VARCHAR(1) NOT NULL 
  ,CODE2 VARCHAR(1) NOT NULL
  ,CODE3 VARCHAR(1) NOT NULL
  ,RATE  INTEGER  NOT NULL
  ,VALUE INTEGER  NOT NULL
  ,MONTH INTEGER  NOT NULL
);


begin
INSERT INTO mytable(CODE1,CODE2,CODE3,RATE,VALUE,MONTH) VALUES ('A','B','C',1,1,202001);
INSERT INTO mytable(CODE1,CODE2,CODE3,RATE,VALUE,MONTH) VALUES ('A','B','C',1,1,202002);
INSERT INTO mytable(CODE1,CODE2,CODE3,RATE,VALUE,MONTH) VALUES ('A','B','C',1,1,202003);
INSERT INTO mytable(CODE1,CODE2,CODE3,RATE,VALUE,MONTH) VALUES ('A','B','C',2,1,202004);
INSERT INTO mytable(CODE1,CODE2,CODE3,RATE,VALUE,MONTH) VALUES ('A','B','C',2,1,202005);
INSERT INTO mytable(CODE1,CODE2,CODE3,RATE,VALUE,MONTH) VALUES ('A','B','C',1,1,202006);
INSERT INTO mytable(CODE1,CODE2,CODE3,RATE,VALUE,MONTH) VALUES ('A','B','C',1,1,202007);
INSERT INTO mytable(CODE1,CODE2,CODE3,RATE,VALUE,MONTH) VALUES ('A','B','C',1,1,202008);
INSERT INTO mytable(CODE1,CODE2,CODE3,RATE,VALUE,MONTH) VALUES ('A','B','C',1,1,202009);
end;
/
1 rows affected

select code1, code2, code3, rate, value, min(month) start_dt, 
    case when row_number() over(partition by code1, code2, code3 order by max(month) desc) = 1 then 999912 else max(month) end end_dt
from (
	select t.*,
		row_number() over(partition by code1, code2, code3 order by month) rn1,
		row_number() over(partition by code1, code2, code3, rate, value order by month) rn2
	from mytable t
) t
group by code1, code2, code3, rate, value, rn1 - rn2
order by start_dt


CODE1
CODE2
CODE3
RATE
VALUE
START_DT
END_DT


A
B
C
1
1
202001
202003

A
B
C
2
1
202004
202005

A
B
C
1
1
202006
999912