Power Query - trying to remove a dynamic column that's created/updated when I access external data source

You can do this with Table.SelectColumns where you use List.Select on the Table.ColumnNames.

= Table.SelectColumns(
      PrevStep, 
      List.Select(Table.ColumnNames(PrevStep), each Text.Length(_) <= 8)
  )

Although both Alexis Olson's and Justyna MK's answers are valid, there is another approach. Since it appears that you're getting data for each month in a separate column, what you will surely want to do is unpivot your data, that is transform those columns into rows. It's the only sensible way to get a good material for analysis, therefore, I would suggest to unpivot the columns first, then simply filter out rows containing the last month.

To make it dynamic, I would use unpivot other columns option - you select columns and it will transform remaining columns into row in such a way that two columns will be created - one that will contain column names in rows and the other one will contain values.

To illustrate what I mean by unpivoting, when you have data like this:

You're automatically transforming that into this:

You can try to do it through Power Query's Advanced Editor. Assign the name of the last column to LastColumn variable and then use it in the last step (Removed Columns).

let
    Source = Excel.Workbook(File.Contents(Excel file path), null, true),
    tblPQ_Table = Source{[Item="tblPQ",Kind="Table"]}[Data],
    #"Changed Type" = Table.TransformColumnTypes(tblPQ_Table,{{"Nov_2020", Int64.Type}, {"Dec_2020", Int64.Type}, {"Jan_2021", Int64.Type}, {"Feb_1_10_2021", Int64.Type}}),
    LastColumn = List.Last(Table.ColumnNames(#"Changed Type")),
    #"Removed Columns" = Table.RemoveColumns(#"Changed Type",{LastColumn})
in
    #"Removed Columns"