The nested flags of containers has nothing to do with nested virtualization. It's if you want to run containers inside of containers (e.g. podman or docker inside lxc ( which is NOT recommended) or lxcs inside lxcs (this should work usually)) or...
it is on by default, per the kernel module settings, but it's not required at all. You only need it to emulate an hypervisor as a VM, so that qemu can itself use KVM and the virtualization extension of the CPU. It's not even available to a VM by...
it will reach the enterprise / pve-no-sub repositories shortly, I'm sure, but getting bits from pvetest is not exactly recommended. You could use pinning to only bring a specific package from that repository (and ignore the rest, or opt-in for...
Mitigation on hosts not relying on nested virtualization is to indeed disable nested KVM through:
options kvm-intel nested=N
options kvm-amd nested=0
in /etc/modprobe.d/nested-kvm.conf