I've owned two Subarus and
I've owned two Subarus and have a theory on what may be contributing to these head gasket issues. My first was a 96 Legacy. I noticed sometime in the first 50K miles that the painted steel bracket that holds the battery down was seriously rusted/corroded with paint eaten off on the bottom. I deduced that current was running across the top of the battery once it got a thin film of grim on it. I cleaned the top of the battery, cut a strip of heavy inner tube and placed it under the steel battery hold down bracket. No more rust/corrosion of the bracket and before selling it I drove my Legacy 247k miles without issue (none - just general maintenance and timing belt jobs). Did the same on my 2003 Forester ... still driving it with 173k miles on it. So on to my theory. The battery bracket is held down with 2 10" long hooked rods with nuts at the top. If current is getting into the bracket, it's also traveling down the rods to the battery "shelf/platorm" which in turn is bolted or welded to he frame. Not far away, the motor is mounted to the frame. I think current is making it's way to the engine block and head and slowly eating away at the the head gasket. Thoughts? BTW, my neighbor just bought a new 21 Forrester,,, if has the exact same steel battery hold down bracket and hooked rods.