Dynamic Unpivoting in Power Query when new dates are added as columns

Your best bet Imran_Isshack is to use Table.UnpivotOtherColumns, like so:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSlTSUUosTkkDUkZAbIqETcA4VidaKQmhyASq0BCKTcE0SFEyqiITJBON4CalgHjlxkVo1hlDTTNWio0FAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Segment = _t, Item = _t, #"1/1/2021" = _t, #"1/2/2021" = _t, #"1/3/2021" = _t, #"1/4/2021" = _t, #"1/5/2021" = _t, #"1/6/2021" = _t]),
    #"Unpivoted Other Columns" = Table.UnpivotOtherColumns(Source, {"Segment", "Item"}, "Attribute", "Value")
in
    #"Unpivoted Other Columns"

You keep the columns you want, like Segment and Item, then all other columns are automatically unpivoted, no matter how many there are.

How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.