I think most of the chip testing facilities like LOADALL eventually got subsumed into the System Management Mode mechanism, triggered by "System Management Interrupts." It's become used by the portion of UEFI/BIOS firmware that's continually running in the background controlling fans and power, and has the ability to run completely hidden from the operating system by design.

This is correct. The functionality of LOADALL now largely happens in the RSM instruction which returns back to normal execution context from an SMI. In modern firmware, before the SMI handler is invoked by the SMM dispatcher the firmware will switch the mode away from real mode and have its own GDT/descriptors. All this needs to be restored on SMI return thus RSM will parse a specially formatted block of processor context to dump everything back.

286 only, in that form; there's similar capabilities in modern cpus but they're so different its not really comparable.

once you toss aside the OS and look at the hardware with "what can i make it do?" there's still some joyous horrors possible. DOS was good in that it was so little OS to throw aside, where it got between you and the hardware at all.