vSAN Witness Licensing
Here this article is to explain the do-able methods that have not been very clear. Before we start, check out the vSAN 6.5 Licensing Guide.
First of all the common way to place the vSAN Witness are normally in a vSphere cluster. So this cluster is already licensed. It doesn't matter if its the lowest Essential Edition to Enterprise Plus Edition. So if you like to run this even using a vSphere ROBO license, this can be done since ROBO is meant for a small site, not more than 25 VMs. Do take note that, using vSphere ROBO, you will not be able to power on more than 25 VMs (assuming your license is 25VMs pack and has not been divided). You cannot combine the license. Even if you have a lot of hosts, you still cannot power on beyond 25 VMs as this is a hard limit.
Now to explain, the vSAN Witness is a virtual appliance, vApp. It is actually a virtual ESXi server. It itself is a virtual machine so it can run on any hypervisor. Even the free ESXi hypervisor.
Can we run the vSAN Witness as a physical host? The answer is yes, however, it is not licensed so you would need to license it even though it is not managed by vCenter. The reason is simple as it will be given support under vSAN licensing. By putting as a physical server, you just make it even able to host other VM other than as a witness. In such, the normal vSphere license would apply.
In summary, I would recommend to host it in the cheapest form using the free ESXi hypervisor to save on cost if you do not have an existing vSphere cluster to host it.