You can support 16bit in long mode via compatibility mode (the bits are still in the Global descriptor table definition https://wiki.osdev.org/GDT https://wiki.

You can support 16bit in long mode via compatibility mode (the bits are still in the Global descriptor table definition https://wiki.osdev.org/GDT https://wiki.osdev.org/GDT as long as the long mode bit isn't set the, Sz bit can be used to indicate 16bit code), just not real mode without using virtualization extensions. The issue was more the way HANDLEs were managed and getting rid of 16bit limitations. (also getting programmers to stop assuming they could safely do things with the upper parts of handles)