Our failure log where members voluntarily post HG failures is here:
You have the last year of the EJ253 H4 engine, and if you search this thread for "2011", you'll find that there were 9 out of 519 total reports that were for the 2011 model year - and the 519 is a low number; some members reported multiple failures in one post:
Subaru Outback - Subaru Outback Forums - Search Results
These are self-reported failures, it doesn't account for the total number sold, failed cars sold or scrapped instead of repaired, etc. It is therefore not a scientific survey. But it does suggest that very late on in this engine series, Subaru got much better at head gasket materials. My model year (2008) had 25 reports, for example, and I'm report number 454 on the list.
My opinion only here: Anecdotally, Subaru got much better at their H4 2.5 NA head gaskets in 2009, right around the time they introduced the PZEV version of this engine in several states which were pretty high sales volume. The PZEV engine carried a 100,000 mile emissions warranty. Lots of failures had been being logged in the 80k-120k miles range, right around the tail end of this new warranty. I don't think Subaru could stand the potential liability for such a high volume of repairs. And finally, they already knew how to not have them fail - the multilayer on the XT had a very good record on head gaskets. This could all be coincidental, of course, but the evidence is pretty strongly correlated here.
What to look for: External oil leakage at the bottom at the block to cylinder head interfaces is very typical; sometimes it's also coolant. Rarely in this engine has it been an internal leak, meaning that if you catch it and repair it, and didn't run low on any fluids, there's likely no permanent damage or lingering effects after repair.