VMware NSX
ESX host prepared as a Transport Node status goes into Unknown or Host Disconnected state due to lost connection to NSX Manager, which is expected when a host is offline due to hardware failure.
1. Disconnect the ESX transport node from VMware vSphere
2. Drag the ESX transport node out of the cluster
3. From NSX manager UI, select: System > Fabric > Standalone Hosts > Remove NSX with Force Delete option