This API is accessed via a dependency on the AppleUSBAudio driver, and is officially documented in the SampleUSBAudioPlugin sample code.
All of a sudden, kexts calling the pluginDeviceRequest function fail to load because that symbol no longer exists. This has not been documented anywhere, and is very likely a bug. I've failed a radar for it under 21367554.
If anyone else has been able to use this function in their kext on OS X 10.11, please let me know.