It's not proprietary per say, however the long, sad road of specialized suspension made for their bikes has many bikes in the world left useless. I currently have two 10 year old S-works bikes that used their forks from 10 years ago that need lower cups that no longer exist to put on a reg fork and there are only a few service places that seem to be able to service them. Specialized isn't in it alone, Cannondale and Trek are both in the same boat with specialized shocks, but all these special shocks that only they use are usually...