NikonRumors reports that Sigma is rumored to start making mirrorless lenses for Nikon Z and Canon RF mounts in 2021. Back in February Sigma was still not sure if they will produce mirrorless lenses for Z-mount.
Viltrox already announced their first full-frame autofocus mirrorless lens for Nikon Z-mount.
Tamron unofficially confirmed that they will also be making mirrorless lenses for Nikon Z-mount.
Via NikonRumors