We've got one such set of APIs here in multimedia-land, they're the "aux" APIs.
The "aux" APIs (auxGetNumDevs, auxGetDevCaps, auxGetVolume, auxSetVolume, and auxOutMessage) are intended to control the volume of the "aux" port on your audio adapter.
It's a measure of how little used these are that when I asked around my group what the aux APIs did, the general consensus was "I don't know" (this isn't exactly true, but it's close). We certainly don't know of any applications that actually uses these APIs.
And that's not really surprising since the AUX APIs are used to control the volume of either the AUX input jack on your sound card or the output volume from a CDROM drive (if connected via the analog cable).
What's that you say? Your sound card doesn't have an "AUX" jack? That's not surprising, I'm not sure that ANY sound card has been manufactured in the past 10 years with an AUX input jack (they typically have a "LINE-IN" jack and a "MIC" jack). And for at least the past 5 years, hardware manufacturers haven't been connecting the analog CD cable to the sound card (it enables them to save on manufacturing costs).
Since almost every PC system shipped in the past many years (at least 5) has used digital audio extraction to retrieve the CD audio, the analog cable's simply not needed on most systems (there are some exceptions such as laptop machines, which use the analog connector to save battery life when playing back CD audio). And even if a sound card were to add an AUX input, the "mixer" APIs provide a more flexable mechanism for managing those APIs anyway.
So with the "aux" APIs, you have a set of APIs that were designed to support a series of technologies that are at this point essentially obsolete. And even if your hardware used them, there's an alternate, more reliable set of APIs that provide the same functionality - the mixer APIs. In fact, if you launch sndvol32.exe (the volume control applet), you can see a bunch of sliders to the right of the volume control - they're labeled things like "wave", "sw synth", "Line in", etc. If your audio card has an "AUX" line, then you'll see an "Aux" volume control - that's the same control that the auxSetVolume and auxGetVolume API controls. Similarly, there's likely to be a "CD Player" volume control - that's the volume for the CD-ROM control (and it works for both digital and analog CD audio). So all the "aux" API functionality is available from the "mixer" APIs, but the mixer version works in more situations.
But even so, the "aux" APIs still exist in the system in the event that someone might still be calling them... Even if there's no hardware on the system which would be controlled by these APIs, they still exist.
These APIs are one of the few examples of APIs where it's actually possible that we might be able to end-of-life the APIs - they'll never be removed from the system, but a time might come in the future where the APIs simply stop working (auxGetNumDevs will return 0 in that case indicating that there are no AUX devices on the system).
Edit: Clarified mixer and aux API relationship a bit to explain how older systems would continue to work.