Really when people say x86 now tho they don't mean that. They really mean the variant introduced with the 386 which has a linear memory model, memory protection, etc. Or x86_64 which is philosophically akin to the 386 but really a new ISA.
So it's really more like mid-80s or early 2000s, not late 70s.
You can't run a COM program today. Not without emulation. Recent PCs can't even run DOS EXE because they're missing the BIOS interrupts most DOS programs use.
UEFI switches the CPU into 32bit v86 mode or directly in 64bit mode and you can't go back to real mode without a CPU reset, which v86 won't allow (you don't have ring -2 privileges) and 64bit mode can't do at all. I don't have a UEFI system, so I might be wrong (I even hope I'm wrong - it would mean slightly more freedom still exists), but from what I read about it, I'm 90% certain it's not possible.
You're confusing several things here. The only x86 processor that didn't allow returning to real mode was the 16-bit 80286 - on all later ones it's as simple as clearing bit 0 of CR0 (and also disabling paging if that was enabled).
Nothing more privileged than ring 0 is required for that.
"v86" is what allowed real mode to be virtualized under a 32-bit OS. This is no longer available in 64-bit mode, but the CPU still includes it (as well as newer virtualization features which could be used to do the same thing).
Even if you could/can it is an anachronism. Architecturally there's just a huge difference between 8086 and even 80286 and the 386. Before the 386 I wouldn't touch a machine with an Intel processor in it. Once the 386/486 penetrated the market and became cheap it was game over for everything else because it was good enough (linear address space, memory protection, larger address space, 32-bit etc etc), smart enough, gosh darn it it was cheap and everywhere.
No, you are wrong. DOS COM files if 16 bit can't be run on 64 bit CPU's but 32 bit DOS binaries can be run under 32 bit GNU/Linux installs with DosEMU straight by just emulating the BIOS part, the rest is native.
Set seg_32bit=0 and you can create 16-bit code and data segments. Still works on 64 bit. What's missing is V86 mode, which emulates the real mode segmentation model.
50/50, because once you boot a 32 bit os you can run 16 bit binaries :)
I'm pretty sure that if I make a dual-kernel 9front (9pc and 9pc64 available at boot) in a 64 bit machine and I compile emu2 for it, DOS COM binaries might be trapped enough to run simple text mode tools under the 386 port.
So it's really more like mid-80s or early 2000s, not late 70s.