I would like to know if the query can be simplified further (or) any other way to rewrite the SQL query

The NVL(CASE WHEN cond THEN val END, 0) is the same as CASE WHEN cond THEN val ELSE 0 END and even further windowed SUM omits NULLS so simple CASE WHEN cond THEN val END:

SELECT id_col,order_col,
sum(CASE WHEN INDEX = 2 AND trim(order_list) != '' THEN order_list END) 
  OVER (partition BY order_col) AS total_qty, 
sum(CASE WHEN INDEX = 3 AND trim(order_list) != '' THEN order_list END)
  OVER (partition BY order_col) AS total_amt,
sum(CASE WHEN order_list LIKE '21235%' THEN replace(order_list,'21235=','')END) 
  OVER (partition BY order_col) AS sav_001,     
sum(CASE WHEN order_list LIKE '21236%' THEN replace(order_list,'21236=','')
     END) OVER (partition BY order_col) AS sav_002,
sum(CASE WHEN order_list LIKE '21237%' THEN replace(order_list,'21237=','')
     END) OVER (partition BY order_col)AS sav_003
FROM  tbl_001  T1
qualify row_number() over (partition by order_col order by date desc) = 1;

Well tiny things first:

trim(order_list) NOT IN('', ' ') if you have trimmed the white space, the second case cannot be. so it can just be trim(order_list) <> ''

which sized, up you possibly could put that into a WHERE clause, (it would eliminate possible order_col rows with just zeros, which maybe you want.

Lukasz's SUM ignores nulls, still holds.

SELECT 
    id_col,
    order_col,
    sum(CASE WHEN INDEX = 2 THEN order_list END) OVER (partition BY order_col) AS total_qty, 
    sum(CASE WHEN INDEX = 3 THEN order_list END) OVER (partition BY order_col) AS total_amt,
    sum(CASE WHEN order_list LIKE '21235%' THEN replace(order_list,'21235=','')END) OVER (partition BY order_col) AS sav_001,     
    sum(CASE WHEN order_list LIKE '21236%' THEN replace(order_list,'21236=','')END) OVER (partition BY order_col) AS sav_002,
    sum(CASE WHEN order_list LIKE '21237%' THEN replace(order_list,'21237=','')END) OVER (partition BY order_col) AS sav_003
FROM tbl_001  T1
WHERE trim(order_list) <> ''
qualify row_number() over (partition by order_col order by date desc) = 1

You are currently building the SUM for every row and then throwing away all but on ROW, and the exist rows care are id_col yet that is never used in any of that OVER()'s, so change to grouping by order_col

old code with some example:

with tbl_001 as (
    select * from values
    (1,10, '10', '2022-03-01'::date, 2),
    (2,10, '11', '2022-03-02'::date, 3),
    (2,10, '21235=12', '2022-03-02'::date, 1),
    (2,10, '21236=13', '2022-03-02'::date, 1),
    (2,10, '21237=14', '2022-03-02'::date, 1)
    t(id_col, order_col, order_list, date, index)
)
SELECT 
    id_col,
    order_col,
    sum(CASE WHEN INDEX = 2 THEN order_list END) OVER (partition BY order_col) AS total_qty, 
    sum(CASE WHEN INDEX = 3 THEN order_list END) OVER (partition BY order_col) AS total_amt,
    sum(CASE WHEN order_list LIKE '21235%' THEN replace(order_list,'21235=','')END) OVER (partition BY order_col) AS sav_001,     
    sum(CASE WHEN order_list LIKE '21236%' THEN replace(order_list,'21236=','')END) OVER (partition BY order_col) AS sav_002,
    sum(CASE WHEN order_list LIKE '21237%' THEN replace(order_list,'21237=','')END) OVER (partition BY order_col) AS sav_003
FROM tbl_001  T1
WHERE trim(order_list) <> ''
qualify row_number() over (partition by order_col order by date desc) = 1

gives:

So swapping to a GROUP BY form, that id_col is a bit yucky.

with tbl_001 as (
    select * from values
    (1,10, '10', '2022-03-01'::date, 2),
    (2,10, '11', '2022-03-02'::date, 3),
    (2,10, '21235=12', '2022-03-02'::date, 1),
    (2,10, '21236=13', '2022-03-02'::date, 1),
    (2,10, '21237=14', '2022-03-02'::date, 1)
    t(id_col, order_col, order_list, date, index)
)
SELECT 
    array_agg(id_col)within group(order by date desc)[0] as id_col,
    order_col,
    sum(CASE WHEN INDEX = 2 THEN order_list END) AS total_qty, 
    sum(CASE WHEN INDEX = 3 THEN order_list END) AS total_amt,
    sum(CASE WHEN order_list LIKE '21235%' THEN replace(order_list,'21235=','')END) AS sav_001,     
    sum(CASE WHEN order_list LIKE '21236%' THEN replace(order_list,'21236=','')END) AS sav_002,
    sum(CASE WHEN order_list LIKE '21237%' THEN replace(order_list,'21237=','')END) AS sav_003
FROM tbl_001  T1
WHERE trim(order_list) <> ''
GROUP BY order_col;

gives (same answer):

So that might perform better, and is more readable to me.

If performance was super critical I would try to see if this improved things:

with tbl_001 as (
    select * from values
    (1,10, '10', '2022-03-01'::date, 2),
    (2,10, '11', '2022-03-02'::date, 3),
    (2,10, '21235=12', '2022-03-02'::date, 1),
    (2,10, '21236=13', '2022-03-02'::date, 1),
    (2,10, '21237=14', '2022-03-02'::date, 1)
    t(id_col, order_col, order_list, date, index)
)
SELECT 
    any_value(id_col) as id_col,
    order_col,
    sum(CASE WHEN INDEX = 2 THEN order_list END) AS total_qty, 
    sum(CASE WHEN INDEX = 3 THEN order_list END) AS total_amt,
    sum(CASE WHEN s_order_list[0] = '21235' THEN s_order_list[1] END) AS sav_001,     
    sum(CASE WHEN s_order_list[0] = '21236' THEN s_order_list[1] END) AS sav_002,
    sum(CASE WHEN s_order_list[0] = '21237' THEN s_order_list[1] END) AS sav_003
FROM (
    SELECT 
        first_value(id_col) over(partition by order_col order by date desc) as id_col, 
        order_col, 
        order_list,
        split(order_list,'=') as s_order_list,
        index
    FROM tbl_001
    WHERE trim(order_list) <> ''
)  T1
GROUP BY order_col;

And I total personally would not use CASE and would use IFF

with tbl_001 as (
    select * from values
    (1,10, '10', '2022-03-01'::date, 2),
    (2,10, '11', '2022-03-02'::date, 3),
    (2,10, '21235=12', '2022-03-02'::date, 1),
    (2,10, '21236=13', '2022-03-02'::date, 1),
    (2,10, '21237=14', '2022-03-02'::date, 1)
    t(id_col, order_col, order_list, date, index)
)
SELECT 
    any_value(id_col) as id_col,
    order_col,
    sum(IFF(INDEX = 2, order_list, null)) AS total_qty, 
    sum(IFF(INDEX = 3, order_list, null)) AS total_amt,
    sum(IFF(s1 = 21235, s2, null)) AS sav_001,     
    sum(IFF(s1 = 21236, s2, null)) AS sav_002,
    sum(IFF(s1 = 21237, s2, null)) AS sav_003
FROM (
    SELECT 
        first_value(id_col) over(partition by order_col order by date desc) as id_col, 
        order_col, 
        order_list,
        split_part(order_list,'=', 1)::int as s1,
        split_part(order_list,'=', 2)::double as s2,
        index
    FROM tbl_001
    WHERE trim(order_list) <> ''
)  T1
GROUP BY order_col;