FWIW, I’m currently trying to get the spice-vdagent branch with Wayland support (merge request 57) working on one of my Whonix 18 guests was able to build it fine but I’m running into issues when I try to actually use it.
I’ve posted the issue here; Replacing spice-vdagent/spice-vdagentd to fix clipboard sharing
If you have any pointers that would be appreciated 
There is also ‘paprika-vdagent’ which is a partial reimplementation of spice-vdagent specifically for Wayland (though it only supports file and clipboard sharing currently, no resolution changes and such) should theoretically work though I ran into a different permissions issue when I tried that, if I don’t get any traction on the spice-vdagent replacement path I may revisit.