Analysis

Your inner SQL statement is always TRUE (for non-null values). You are probably wanting the max(version) for each project_id, not each version.

Additionally, these scalar sub-queries are known to be slow at times.

Solution

Use an Analytical function.

PLAN vs

Yours, right side, is doing two Full Table Scans vs my one. This becomes worse with larger tables.

Code

select t.project_id,
  t.version,
  t.stp,
  t.t_id,
  t.status,
  t.mp,
  case
    when t.version = max(t.version) over (partition by t.project_id)
    then 'Y'
    else 'N'
   end Is_last_version
from t_proj_f t

code review note: always use the appropriate table alias on ALL columns, especially if the column could be misinterpreted.

I think this will give you want you want:

 select project_id,
        version,
        stop,
        t_id,
        status,
        mp,
        case when rowno = 1 then 'Y' else 'N' end is_last_version
        from (
              select project_id,
                     version,
                     stop,
                     t_id,
                     status,
                     mp,
                     row_number() over (partition by projecT_id order by version desc) 
                        as rowno
              from t_proj_f t
              order by project_id)