If we're going to change to Off Proxy Mode, what is then required?
You just need to enable the proxy role on physical machine and choose the proxy in the job settings.
In 2016 is it still needed that the hosts running as Off Proxies have the Hyper-V role installed?
Yes, Hyper-V role is needed both for VMs which you want to use as off-host ones (not a recommended approach) and for physical machines.
Here is the full list of requirements.