Best answer by Jon
That will expose typical nested virtualization support.
Note: Enabling nested virtualization precludes that specific vm from many features, such as ADS and live migration. It also precludes the cluster from doing any sort of rolling maintenance that would require live migration.
i.e if you had to upgrade the hypervisor or BIOS, you'd have to shut down the VM with this flag enabled.
This is because KVM/QEMU based systems, just at general technology level, do not support live migration for nested virtualization enabled VMs. Google apparently just patched this for GCP but that code hasnt made it upstream yet.