I was just wondering whether anyone else has managed to publish from an earlier version of Desktop to ArcGIS Server. Reading the ESRI documentation this shouldn't be possible but whilst doing some testing using Desktop 10.3.1 with AGS 10.4.1 I have managed to create a very basic model and publish it as a geoprocessing service. I have then run this GP service in Desktop 10.3.1 and it works and have also added this service to a widget in Portal 10.4.1 and it runs fine. I am an administrator in Portal.
There are no actual blocks to prevent this from happening. And, as you've discovered, there are times when it may work. You're welcome to do it, but as you've read and know, the workflow is unsupported. Meaning if you continue in this configuration with GP Services and you experience something unexpected, TechSupport cannot help and you might not be able to do what you really want to do. We have a goal of closing this gap: meaning either the software will actually block the mis-matched versions + gp service publishing, or we allow the mis-matched versions but ensure the service you publish will work. Naturally the 2nd option would be the more desired one, but I cannot speak to when and what direction we'll go.
Great, thank you for your prompt response. The second option would definitely be more desirable. Upgrading desktop for the whole organisation can be a long process whereas server can be upgraded relatively easily and obviously we're always wanting to take advantage of the new functionality in Server/Portal. Thanks again for the clarification.