When I first got my Subaru in 2012, I was immediately told by fellow enthusiasts to wave at other Subarus on the road. The "Subaru Wave" seemed like a great random act of camaraderie between all owners, especially to someone new like I was. Shortly thereafter, I learned that not all Subaru owners wave back... or even notice! Unless I was at a meet or I personally knew the owner of that car, the "Subaru Wave" seemed like more of an ideal than something that was actually practiced.
Nearly every forum, website, blog, subreddit, or social media outlet for Subaru enthusiasts has a handful of "I waved to a (insert name and detailed description of a Subaru) and didn't get a wave back!" posts noting this. There are entire threads on NASIOC devoted to "The Subaru Wave". From detailed rules about how to wave to general complaints that no-one waves back, it seems like there's an awful lot of owners who at least know about the wave, but likely have given up due to the growing lack of reception from fellow owners.
Aside from enthusiasts in WRXs and STIs, there's not many owners who know, or even care to know, about this gesture. It's not the end of the world, but really just part of what happens with a growing brand like Subaru, as things are no longer like they were with the tight-knit community before. You can still find this camraderie at Subaru meets and events, but there's something special about getting a wave, still. It happens when you're out and about on a bland commute to work that you've driven at the same time every day for eons. Then a random '02 WRX Sedan rolls up that you don't recognize to throw you a wave. In moments like that, "The Subaru Wave" has somehow managed to retain it's charm... that it's not something you get every day. It becomes a sign of the enthusiast who hasn't forgotten.