However, my experience is that these oil leaks were common to this engine.
not any more common than any other engine.
those websites are generic oil seal replacement guidelines and leaks. i can find oil seal leak examples for any other make/engine...toyota, nissan, honda...they leak too. there's nothing specific suggesting "this engine" is any more prone to leaking than any other. a properly installed seal always lasts longer than a couple of months. if it doesn't then the seal was compromised, out of spec, or not installed properly.
not trying to argue, just trying to help you. either something was wrong with the engine - it's very common for people to scar the sealing surface while trying to remove a seal - have you ever removed one? they can be very tricky to remove in confined spaces. this is far more likely. or the mechanic messed up the install, the seal was bad (aftermarket? - though EJ aftermarkets generally seem well constructed, it's usually older generation subaru's that seem to have lower quality aftermarket seals). etc.
go visit any other car forum and you'll see similar issues and recommendations. folks typically replace seals with a timing belt job because they are prone to leakage over time/mileage - true of honda's, toyota's, subaru's - nissan's, etc. this is not EJ25 specific....nor even subaru specific.
subaru's have twice as many heads as some vehicles - so twice as many cam seals and valve covers in some instances - so maybe they have "twice the propensity" to leak if you want to look at it that way - but still a low percent, and they dont' have "repeat" or low mileage failures if properly done. something is wrong if that's the case.
subaru oil leaks can be really annoying because of the layout of the engine, the exhaust manifold can easily get dripped on, where it burns off, smells bad, and smokes it doesn't just hit the ground like some other layouts. so they can "seem" worse than others sometimes when they're laying out a james bond smoke screen.
also, subaru's actually last long enough and retain value to the point that engine seal wear and leakage becomes an issue. some makes (like ford cars) don't have that "problem". they're generally not worth repairing once they reach ages/mileages where seals start to leak. LOL