I have got the answer by doing this,

First I inserted rank of my status change by ID

 If([Status Change Date] is not NULL,Rank(RowId(),"asc",[ID]))```

then I inserted onw more calculated column to get the last status change date using rank

Last([Status Change Date]) OVER (Intersect([task_id],Previous([Rank of Status Change Date])))

This gave me the Last Status Change Date