is my 1999 impreza outback sport 2.2L (135,000 m) at risk for the head gasket problems commonly associated with some older subarus?

not likely, the 2.2L enigines have very few issues with head gaskets, usually it's on the late 90's early 2000's 2.5L where you start to see head gasket issues become more common.

Nice that defetive materials don't last. They are switched or recalled immediately to avoid litigation.

I have a 2001 2.2 L and a 1997 2.2 L with 205,000 and 195,000 respectively without any head gasket issues. You should be fine for a long time so long as you are keeping coolant and oil levels appropriate in the car.

No, the 2.2 does not have the headgasket issues found in the 2.5 liter engines. (and a bonus, the 2.2 is NON interference, unlike the headgasket blowing 2.5 liter). Properly maintained, the 2.2 is an honest 300k mile motor.

Actually, only the phase 1 2.2 is non interference. The phase 2 2.2 made in 98-99 on up are also interference engines.

Dogbone-1999 2.2 is in fact an interference engine...I have a customers 1999 Outback with 2.2 engine in my shop now with a blown head gasket. I haven't started the work on it yet so I'm not sure what side it is. The coolant disappears and when running there is steam coming out the exhaust. They said they tried a bottle of Barrs Leaks coolant sealer with no luck so they took it to me for replacement. A couple of weeks ago I replaced the leaking radiator on this car so they probably ran out of water and severly overheated the engine causing the head gasket to fail...