Mac OS 8.6
Released May 10, 1999, Mac OS 8.6 added support to the Mac OS nanokernel to handle preemptive tasks via the Multiprocessing Services 2.x and later developer API. But there was still no process separation; the system still used cooperative multitasking between processes, and even a process that was Multiprocessing Services-aware still had a portion that ran in the blue task, a task that also ran all programs that are not aware of it, and the only task that could run 68k code.
Still, this free update for Mac users running 8.5 and 8.5.1 was faster and much more stable than either versions of 8.5.x, and is by some considered the most stable Classic OS. It was also the first Mac OS to have the OS version displayed as part of the startup screen. Many hardware upgrades require a minimum of Mac OS 8.6.
Read more about this topic: Mac OS 8