Raspberry Pi 4 defies the keyboard.
I'm not sure what you mean by defies in this context? They keyboard
doesn't work? At what point are you having issues. There is no support
for USB (actually the PCIe), hence keyboards, in the U-Boot firmware
which means it won't work in grub or the early boot process. Once it
gets to Linux such as the login prompt they keyboard should work find
as there is USB support.
I have been under the assumption that 5.6 has support built-in? Any
Support for what exactly? Support for a device such as the RPi4 is not
a binary thing. There is initial support and enablement so it's useful
for a number of situations but the support is far from complete and
hence while for a it works for a lot of use cases we don't officially
support the RPi4 yet because the HW enablement is incomplete.