When did Porsche stop producing air-cooled engines in the 911?

Context: The Porsche 911 is a two-door 2+2 high performance rear-engined sports car introduced in September 1964 by Porsche AG of Stuttgart, Germany. It has a rear-mounted flat-six engine and originally a torsion bar suspension. The car has been continuously enhanced through the years but the basic concept has remained unchanged. The engines were air-cooled until the introduction of the 996 series in 1998.

Answer: Porsche stopped producing air-cooled engines in the 911 in 1998 with the 996 series.