Good Morning @gareth_finney
I have not received any confirmations from ESRI. But from what I've read, This has been added in a later version of Arc Enterprise, 10.9.1, https://enterprise.arcgis.com/en/sites/10.9.1/content/data-download-settings.htm . Doesn't look like this will ever reach 10.6.1 unfortunetly.
What I have ended up doing is setting up a Collaboration between Arc Portal and AGOL, this way, people can download data that is hosted in our Production Portal. I created a download site within AGOL that is linked on our main site in Portal. Hope this helps! If you have anymore questions, feel free to ask.
John
