It seems the Nikon Z 8 is reality, and already used by a Nikon Ambassador.…
EDIT: appears to be a design mock-up, see the comment section.
Mirrorless Rumors posted an image of what might well be Nikon’s upcoming entry level full frame mirrorless camera.
So far it’s not entirely clear if the image above is a mockup/fake or if it is real. So take it with a grain of salt.
Given that Canon already has an entry level full frame mirrorless camera, the EOS RP, it’s likely Nikon will soon follow up with their own model.