have there been any sudden acceleration issues on 2008 Subaru Legacy

how fast must car be going for airbag to deploy have there been any sudden acceleration issues

The airbags can deploy at any speed if the impact sensor is struck. I have not heard of sudden acceleration problems and just looked there is no recall but check this government website
2 more answers , 1 more comment
I am not aware of any sudden acceleration issues with Subaru's. Be sure to check our "Car Info" section about your car to research further.

Air bags - It's not so much about the speed you must be going to deploy the air bag -- it's more about the inertia energy that is created by whatever accident occurs. Think of an air bag sensor as a magnet with a metal ball secured to it. When the inertia is enough to knock that ball off the magnet - that's when the air bag deploys (in basic terms).

A small bumper collision with minimal damage probably will not deploy the bags. Something with a bit more impact - like 5-10 mph against something moving 5 mph could.

To sum up - there is no specific speed that determines deployment. Hope this helps.
The newer the car, the smarter the SRS system. Low speed air bag deployment is rare. As far as sudden acceleration, it would be a good idea to have a technician check the IAC valve and throttle housing for carbon buildup, this valve controls air flow on decleration (When your foot is not on the gas pedal). If there is an excessive buildup, a throttle service should be performed at the very least.
Also make sure the gas pedal is clear, no floor mats getting in the way.