BeOS was definitely a real time OS. BeOS had two scheduling classes, threads with a priority level between 1-99 were time shared threads where the quantum was proportional with 2^priority_level, threads with a priority level between 100 and 120 were real time threads, they could preempt any other thread, including any kernel thread, they were locked to a CPU and never preempted. And when a real time thread was created, it preempted any time shared thread immediately, the scheduler run at creation, it wasn't only added to a scheduling queue like time shared threads.

QNX is a still very popular realtime OS in the embedded world today. Btw, the OS run by the BlackBerry devices is QNX. Usually you'll find QNX in control systems like avionics.

From what I've read, "realtime" threads with higher priority could preempt other realtime threads, meaning they could be pre-empted.

That's true of every other RTOS out there, in fact, it's required to be this way in order to provide deterministic latency.