Their departures follow that of manager Kevin Keegan last Thursday.
McDermott had been taking first-team coaching since early last week when the differences between Keegan and the board that led to his resignation became apparent.
Terry McDermott has left Newcastle as assistant boss
McDermott, a former Magpies player, had a long coaching association with the club dating back to Keegan's first stint in charge in 1992.
He also worked under Kenny Dalglish, Graeme Souness, Glenn Roeder and Sam
Allardyce at St James' Park.
Former youth team goalkeeper Sadler worked at the club's academy until being promoted by Roeder in 2006.
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