Shout out to Windows Real Mode, which had a built-in virtual memory manager that swapped out unused memory to disk. All of that required apps to participate, of course, but it could multi-task in the lower 640K.
Astonishingly, Real Mode still exists even on 64-bit Intel chips, and is still the default mode. Booting a modern OS involves a whole stack of bootstraps to jump from Real Mode to Protected Mode to Long Mode.
Real mode is finally becoming a vestigial feature; some modern EFIs have a tiny assembly stub that immediately enables protected mode; from there EFI, the boot loader, and the OS can all be 32/64 protected mode programs.
That's one of the things I feel FOSS is largely underestimating, the amount of old code running in offices around the world. You can still run programs from the Windows 1.x-3.x era (win16) if you have a 32 bit Windows 10 install. You apparently need the 32 bit version because 64 and 16 bit modes are mutually exclusive (or some such).
It's because Virtual 8086 mode is unavailable in long mode. So either you would have to switch the cpu in and out of long mode (which probably has a performance penalty and complicates the code), or use the more general purpose virtualization support (which may not be available), or just emulate everything. It not surprising that Microsoft just decided that it wasn't worth it.
