From the changelog:
macOS host: Dropped all kernel extensions. VirtualBox relies fully on the hypervisor and vmnet frameworks provided by Apple now. At the moment the implementation lacks “Internal Networking” functionality. This will be provided at a later date.
Unable to test but “Internal Networking” is what Whonix uses to communicate between VMs. Until the functionality is implemented, the 6.1 branch is still available and will be supported until December 2023.
(I wanted to provide links but the system won’t let me.)