Let's make spice-vdagent a "weak recommended package"?

Similar to virtualbox-guest-x11. See:
https://github.com/Whonix/Whonix/blob/master/build-steps.d/1700_install-packages#L389

Let’s make spice-vdagent a “weak recommended package” so it can be easily purged and/or not installed by non-KVM targets?

Sure. And BTW there is a a related xserver package needed for qxl too for higher resolutions and 2d acceleration. If you want to make it a weak dependency go for it.

Great.

Done:

Please check if spice-vdagent will be installed in Whonix 10 release when there are testers-only versions, but I don’t think there will be issues.