You can create a named range for L:T. After adding new columns of named range shifts.
Then you can use it as follows:
Property Get slp_hide_col() As String
slp_hide_col = "myNamedRange"
End Property
and then change Columns(slp_hide_col)to Range(slp_hide_col):
Public Sub SLP_Config_Click()
With Range(slp_hide_col)
.Select
.EntireColumn.Hidden = Not .EntireColumn.Hidden
End With
End Sub

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