The notable producer absent from Busta Rhymes' album 'Back on My B.S.' after 11 years is Dr. Dre. This conclusion is based on the information from Wikipedia's page on 'The Big Bang' album, which states that production for that album took place during 2005 to 2006 and was handled by several producers including Dr. Dre. However, the search results for 'Back on My B.S.' do not mention Dr. Dre being involved, indicating his absence after the long-standing collaboration.