Linux actually has pretty long history of userspace drivers, and multiple kernel drivers have as their main if not only logic a way to link hardware interface w

Linux actually has pretty long history of userspace drivers, and multiple kernel drivers have as their main if not only logic a way to link hardware interface with userspace app, or just providing transport interface. And android heavily uses such interfaces from day 0, which is why you have separate binder instance on Android for drivers. As for Binder being C++, no it isn't. Historically it has connection with C++ due to how it was used, but like other IPC it's not bound to C++, and arguably given how it is structured compared to some other options there's even less C++ specific stuff in it. Also, IIRC the use of Binder predates acquisition of Danger (?) by Google