OpenGL 4.2, backwards compatible to OpenGL 1.0 (1992)
OpenGL ES, not backwards compatible with OpenGL.
There's an obvious parallel here of APIs being compatible within themselves but not across boundaries between major architectural revisions intended to throw out cruft and target new environments.
That's why it's called X11 and not X10 ES.
OpenCL is a different API from OpenAL, Cocos-2d is an API for an entirely different language than Cocos-2d-x, the Cocoa API is wildly incompatible with Cocoa Touch. Horrors!
You determine whether or not two APIs are compatible (or even striving to be the same kind of API) by reading the documentation, not by applying stupid heuristics to common substrings in their names.
Side-note: Win16 (called "the Windows API" back then) actually predates X11 (though not previous versions of X) since Windows 1.0 was released in November 1985.
What I'm wondering is this: Has there ever been an API with a longer deprecation period than Win16? Remember, Microsoft announced the deprecation in 1995 (IIRC), but it didn't start to bite until 64-bit Windows mattered (you could draw the line at some server versions, Vista, or, as I do, at Windows 7). No matter how you slice it, that's a long time.