Unknown whether it will come back by way of some hook into NVidia's API, but currently it is disabled for a very specific reason:
The Nvenc (new) encoder performs the entire process from render to encode without sending any data back over the PCIe bus. This comes with serious benefits (like freeing up bandwidth if you're dealing with limited lanes available), but also comes with the restriction that rescaling is not possible, as that currently requires the frame buffer sent back off the GPU after render, then resized and sent back to the encoder.
If you need the rescale option, you'll need to switch off the Nvenc (new) encoder for now. (small note: there was a version where it appeared as though the rescale options was available for the new encoder, but that actually silently switched to the old encoder to perform the resize)