It could be this is done by MacOS itself, in a similar manner to Hyper-V on Windows, where you can no longer use VMWare/VirtualBox/etc. Virtualisation is critical to Win 10 security, wouldn't be surprised to see MacOS going in that direction.
New VMware and VirtualBox versions can rely on hyper v platform so you can use them with vbs enabled
Yes, if it worked. But unless they fixed it recently the Virtualbox implementation was super buggy/unusable.
My Windows used to BSOD when running with hyper-v enabled. I disabled it and I'm not going to enable it again anytime soon. VirtualBox works just fine. I don't know why do you think that it's critical to Win 10 security.
And at least on my machine, hybrid sleep doesn't seem to be compatible with Hyper-V, either...
It is in newer versions of Win 10, google Credential Guard and Device Guard.