This finds 168 matches:

Option Explicit

Sub Danielc(): Dim Sku As String, wd As Worksheet, ws As Worksheet, r As Long
Set ws = Sheets("Source"): Set wd = Sheets("Discontinued Items")
With CreateObject("Scripting.Dictionary")
For r = 2 To wd.Range("A" & Rows.Count).End(xlUp).Row
Sku = Trim(wd.Cells(r, 1)): .Item(Sku) = r: Next r
For r = 2 To ws.Range("F" & Rows.Count).End(xlUp).Row
                Sku = Trim(ws.Cells(r, 6))
If .Exists(Sku) Then ws.Cells(r, 7) = "Yes" Else ws.Cells(r, 7) = "No"
Next r: End With: End Sub