ArcaOS 5.1, which is an OEM version of OS/2, doesn't require CSM, though it is recommended to enable it as it helps set up the hardware for a 32bit OS, things like having the framebuffer below the 4GB barrier. Also maximizing memory, a lot of modern machines only leave a couple of GB's or less available below 4GB
And while you don't need ring 2 access, you do if you want to use DOS programs, including Win3.1 and of course it needs ring 0 access for the kernel and device drivers.
Biting the hand that feeds IT
