The default kernels do not support "Power off on halt".
This can be found under APM (Advanced Power Management) in
the kernel config menus.
Most modern PC's use ATX power supplies, which support this
feature nicely. Even though APM is mainly laptop-oriented,
I can think of no reason not to have this in the default
kernels. Worst case an AT power supply will ignore the
poweroff message(?), and possible error messages would occur
on non-conformant machines.
this has been changed in the kernels shipped with 5.9.x.